There are three factors which come into it.
1) Oxford and Cambridge are small, boring towns. University is about socialising and exploring in every possible way. Thus, said people wished to go to more lively towns, which also have a good reputation.
2) Some of said people had lived in Oxford previously and wanted to move away from home. This, combined with the incredibly boring nature of Oxford, made me not even begin to think about attending either Oxford or Cambridge - not that I would have necessarily got in of course.
3) The course at Oxford was not for them, and they wished to take a course which interested them more at another well regarded University, such as Warwick.
These all seem like excellent reasons not to apply to Oxbridge - but it would seem bizarre for someone to have these opinions and then still go to the considerable effort of turning up for several interviews and admissions tests!
I'm certainly not going to suggest your friends are fibbing, but it's very rare for people to apply, come to interview, get an offer and then turn it down (as the official admissions reports indicate). So your experience of knowing several people who've rejected an offer from either Cambridge or Oxford is a slight anomaly. [A small note: you mentioned you knew people who turned down Oxford
and Cambridge, which would have course be impossible since undergraduates can only apply to one or t'other. I assume I've misunderstood what you wrote though.]
Sadly many,
many people find it much easier to say they got an offer and turned it down, rather than say they didn't receive an offer after interview. This is very understandable, and especially when many of these stories arise when people first meet at a new University when people are trying to assert themselves academically. The
vast majority of people who apply to Oxbridge get turned down, and many of these people are extremely smart and it's the first time in their lives they've met rejection/failure of any sort. As such, there are some strange reactions and low-and-behold, every Tom, Dick and Harry knows loads of people who turned down offers from Oxbridge.
What irritates me about the fibbers is their actions undermine those who apply and are happy to say "I gave it a really good shot, but didn't quite make the grade". If you're going to apply, you need to live with the possibility that there might be people out there better than you at your chosen subject. Even worse than the fibbers, are those that make out they
could have gone to Oxbridge had they chosen to do so. Such people rarely use the phrase "I could have
applied to Oxbridge". This comment isn't half as strong: it's most likely that if you apply, you'll get rejected.
And breath... personal rant over. I
can't stand people who fib about this sort of thing.