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The most important reasons I can think of are private reasons. People do not live to work, but work to live.

What good does a job at a highly ranked university do, if your family/your friends can not live there or are unhappy to live there?
What good does a job at a highly ranked university do, if it is in a place you find boring or don’t like?

I would suggest you to change your mindset from highly ranked university to high quality of life (which is subjective and also includes a little bit the rank of the university, if you value this).

Adding to the family and personal reasons mentioned, I would like to point out some professional reasons as well;

• High rank universities tend to be bigger, which means they might be busier, impersonal and more stressful.




• A smaller university, (probably of a lower rank), would have a smaller load of students which means the relationship and interaction between professor and students is more direct, more personal and surely fore some, more fun.




• Being part of a lower rank institution can be a challenge for the professor to add significant value to it and try to improve its reputation.




• And finally yes, there can be tons of other reason such as contributing to a particular local community, living in a smaller and more quiet town, more nature closeby etc. etc.

The question seems to imply that university ranking is the most important component or aspect of a university career. I think many would disagree with that. In fact, there isn't any single feature of academia that is of primary concern to everyone. You can't in general list the various positive and negative aspects of academia on a linear scale that works for anyone but yourself.

Sadly, a lot of questions here seem to make the same sort of assumption. Students want to go to the top university in their country, when it might be better just to want an excellent education.

However, such things as the ranking of a university come with both good and bad aspects. You can brag to your mom, of course, that you are teaching at Harvard or Cambridge. But your life will be pretty hectic and there will be quite a few things that you are expected to do that may not appeal to you. The ranking, of course, is there because everyone is working very hard all the time and take few personal breaks with pressure all the time to do even more. The rankings don't just happen.

So, the title of the question is a bit off. There is no "typical" here, other than perhaps that people desire a change of some sort. But what sort of change is a personal thing. Some want to be a bigger fish in a smaller pond. Some want to take on admin responsibilities not open to them where they are. Some want to move to Montana because of the fishing. Or maybe to Florida.

Your academic career will be a mix of things. Some good and some not so good. If the good outweighs the bad you will probably stick with it. Unless you get a desire for a change.

Money

It may not be the only driving factor, but I am surprised to see a lack of people mentioning that just because they are going to a lower ranking university doesn't mean they are being paid less.

I would be willing to be a reasonable percentage of professors moving institutions have been offered a better salary. It is quite common across all career paths these days that in order to get the best salary you need to change employers.

Particularly if you have been working at one institution for a long time the raises you receive may not be inline with the actual increase in your value due to your experience.

Malo hic esse primus quam Romae secundus

"Better to be first here than second in Rome." _{Julius C. Caesar (according to Plutarch).}

In many way, prestige, influence, freedom to choose research lines and implementation times, workload level, collaborators, quality of life, sunshine hours per year ...

The answers about personal reasons are excellent and worth a read.

But also remember that metrics are never neutral: any ranking system inherently makes choices about what is important in order to turn a school into a single aggregate number.

Many rankings prioritize things like funding, selectivity rates, peer evaluations, publicity, etc. Some professors may have different weightings, such as:

• Teaching over research (not everyone becomes a professor to write grants!)


• Access to specific collaborators within their subfield (lower ranked school, but strong in one specific area)


• Joint appointments with nearby facilities (national labs, industry collaborators, etc) that boost the attractiveness of the offer


• A desire to support certain student populations that are, uh, not well represented at some big programs


• Ability to find a good job in the same area as their spouse (who may or may not be in the same field, much less work at a university at all)


• Specific terms of the offer that their original host is unwilling or unable to match (more space, director of a new program, etc)

There is an old census joke that "the average family has 1.9 children, but all the families we checked only had whole integers". Faculty are the same way: the average "best" ranking is not a good way to describe an individual.

Among the academics I've known who have done that, the most common reason is that they are going from a university that is good all-round to one which is doing interesting work in their particular sub-field of interest even if it is lousy in everything else.

Those who like teaching also tend to like smaller cohorts, because they can be personally involved. Apart from the really top-end sandstone institutions, the higher ranked universities tend to have rather low academic staff-student ratios and education can become an afterthought. The smaller universities also tend to end up with more senior staff actively involved in research, which is generally considered a lot more fun than administration.

I also know one who switched because he was in love with a postdoc at that lower-ranked uni.

Aside from that, wages matter, and not just headline cash sums. A nicer town/city or a lower cost of living, better opportunities for one's spouse, and so on can all make somewhere else more attractive.

Here is an example of a black mathematician who was a full professor at Purdue, but he recently left for Pomona because he felt racially isolated and excluded.

Turn the question around: what are the benefits of the highly-ranked school? Maybe those don't apply (anymore).

For example, ease of gaining grant money and attracting good graduate students. But if a professor has had a reasonably long and successful career, they may be able to do these things from anywhere.

A second point is that professors often care more about colleagues in their research area than overall prestige and ranking.

Adding to the other answers here: It might very well be that a professor does not give any value to such rankings, maybe because they know on what data the ranking is based on. From my time as a student representative in Germany (Fachschaftsrat/Fakultätsrat), I remember rankings where the data for some universities consisted of around 10 survey returns...

Therefore, they might base their decision on personal reasons, on the faculty-to-student ratio, on the staffing and research facilities they are offered or maybe even on an upcoming research project. Or maybe they want to conclude their career on a chair at the same university they currently are at -- at least here in Germany, climbing the professoral career ladder is harder if you don't switch to another university (in my universities' senate, which I was part of for a while, such "Hausbewerbungen" (applications from researchers of the same university) warrant a very sceptical look onto the candidate list).

Yet another possible reason: they might have been head-hunted.

A somewhat singular example of this, dating back to 1964, before the quantified ranking of universities emerged, was at the nascent University of Warwick. Had universities been ranked then, Warwick—as a complete unknown—would have been assigned a pretty poor score. [Sir] Christopher Zeeman, charged with setting up Warwick's mathematics department, invited six distinguished researchers to fill the initial positions, the lure for each being that the other five (plus Zeeman) would be there. The bluff worked, and the department went on to be a cornerstone of what is now a highly ranked university.

I think an observation I made while applying to both high and low ranked universities could go a little way to explain this phenomenon; I have found that low ranked universities treat students and staff very well in order to encourage good academics and students to remain there and Improve the uni’s ranking. While high ranked unis don’t have to try so hard as there is always a surplus of student and quality staff applications.