Everyone is overly studious and the expectation at the uni is that you work almost non-stop. I met with my tutor to discuss motivation as I was finding the work load too much and it was getting me down. He told me it was all about reducing the number of "ineffective hours" in a day which he described as hours in which one is not studying.
He said that many students would go home and spend a couple of hours a night watching TV or socialising (i.e relaxing) and that's why they weren't doing as well as they should do.
The teaching staff have a very mechanical view of life and assume that you want nothing more out of life than a degree from Imperial. Also the lack of humanities subjects creates a strange atmosphere; not just the lack of females but also the general type of student that excels in Maths based subjects.
Add to the general lack of emotional intelligence of STEM students the big egos they carry with them from being one of the best students (if not the best) in their previous school and you get a bizarre student population.
If you are a home student who enjoys a social life (going out drinking/clubbing) your options will be limited, the number of overseas students who treat an Imperial degree as a job really limits your options. This means that you may find only one group in your halls that frequently goes out (once every one or two weeks).
I assume people at Imperial are dissatisfied with life due to the high expectations that they have imposed on themselves. There is a 6 month waiting list for student counselling services and if you are not literally on the brink of suicide or self-harm they won't offer more than two sessions.
As for it being "fashion" or "a thing" for Imperial students to dislike Imperial. If you are someone who has enjoyed a fairly balanced life with more to life than school work you are most likely going to dislike Imperial and feel fairly out of place there. There will be people you get along with, however it may not be as easy to find friends as it would be in another university.
Saying this, I myself have forged a decent student life, but my best times have come away from Imperial and a sense of dread still fills me each time I step foot on that campus. I am scraping through my degree and not a day passes that I don't think of dropping out or what could have been if I went to a different university.