I would guess its a combination of many things (most have been mentioned here) from the over elastic collisions (not enough energy is disipated on impact?), the way the physics engine operates to the highly tuneable setups we can have (which enables us to have setups perhaps not possible on similar cars in RL?) but I think that the setups we tend to go with and the fact that we as virtual drivers will go to the edge without any fear as real life drivers have is much of the reason for these flipping occurances
I'm reminded of a particular qualification session at Bathurst with the V8Supercars and a driver named Greg Murphy... In one particular year he did an absolutely awsome lap about 1 sec faster than any of his competitors in a class of racing where 100th of a sec is alot (he got a standing ovation from the whole pit lane) and I put it down just too his pure GUTS (or stupidity ) to drive at the very edge for the entire lap, his car was on 2 wheels quite alot on that lap I think we as virtual drivers drive much closer to the ragged edge than we probably would in RL...
I'm reminded of a particular qualification session at Bathurst with the V8Supercars and a driver named Greg Murphy... In one particular year he did an absolutely awsome lap about 1 sec faster than any of his competitors in a class of racing where 100th of a sec is alot (he got a standing ovation from the whole pit lane) and I put it down just too his pure GUTS (or stupidity ) to drive at the very edge for the entire lap, his car was on 2 wheels quite alot on that lap I think we as virtual drivers drive much closer to the ragged edge than we probably would in RL...