When ever people hate 'groups' its because they have seen a bad example of that group.
I've worked with a lot of Oxbridge people and found them fine. Every now and then you come across one with a slight superiority complex but that's par for the course whenever you are dealing with something that marks out high achievement. When I was at school there were a few lads who were on the youth books of professional rugby or football clubs, a couple of them swanned round like they were internationals already. On the whole most Oxbridgers in person are fine.
But when it comes to the internet, Oxbridge comes over badly because there are a lot of trolls (who may be from Oxbridge or may be just 'walts' pretending they are) who troll in the name of Oxbridge. I suspect that mostly these guys are not from Oxbridge, I can't really see proper 'achievers' spending their time trolling forums, but the internet does bring out the nasty side in the odd person who wouldn't have the balls to actually rub their "Im at Oxbridge" bragging in peoples faces in real life, but with the anonymity of the net they can do.
I've seen a lot of situations on TSR where someone will ask a question, and even though Oxbridge isn't relevant to the situation, somebody allegedly from Oxbridge jumps on the thread, telling the person that they are at a mediocre university and won't be able to get a job because they are at a mediocre university. Of course they are waiting for somebody to respond to them, so they can retaliate with..."What??? You're saying Oxbridge DOESN'T matter? Well here's my evidence...in career x, 90% of the new recruits are from Oxbridge, in league table Y, Oxbridge comes here and your university comes there". They have set it up to derail the thread with an argument about "does Oxbridge matter" and for all the Oxbridge trolls this is a good argument because they can always produce evidence showing that Oxbridge are good.
It's the equivalent of a Man United fan finding some Brighton and Hove Albion forum and going laughing at them saying "haha so you're all excited about being top of League 1 eh?? when did you last win the Champions League? Here's a list of all United major trophies!" The Brighton fans will just think he's an annoying prat and this is what happens with the Oxbridge trolls on here.
Recently I've seen the Oxbridgers have been on the receiving end of a few Ivy League trolls who have been coming on to tell them how much better the Ivy League is than Oxbridge and how internationally, it's well known that Oxbridge's workload is a walk in the park compared to the Ivy League, most Oxbridgers wouldn't come near the Ivy League, and so on.
Having said this, that sort of internet uni snobbery is by no means confined to Oxbridgers. On TSR I have seen worse snobbery from (a few) people at the unis usually associated as Oxbridge reject destinations (Bristol, Exeter, Durham, Bath, Nottingham, UCL and so on), some of these are even more keen to reassure themselves of their university's superiority over others, so you get the odd one talking about their uni like its Oxbridge, to other Russell Group students. I have to laugh when there's an argument and somebody says "well I'm at a TOP TEN university" and starts going on about it. What usually happens with these types is they have been on the IB forum asking about how to get into IB and have got short shrift from the Oxbridge and LSE crew telling them they are only at semi-target unis, so they feel inferior, so they troll other forums to try and do the same to other users.