The cause of the rollovers is simple: One wing breaks away at the wing root.
During the accident investigations of the '97 (FedEx) and '99 (Air China) rollovers the NTSB looked very closely into the forces and strength of MLG and main wing spar. Using DFDR data they where able to calculate that the forces where beyond design loads and safety margins. - Therefore the wing "was allowed" to break regarding the construction standards.
On the other hand other aircraft are much more stable: The 346 in Quito (overrun in 2007) "touched" down with > 3G and sustained no structural damage (apart from the Boogeys and wheels) until it overran.
When BA038 crashed in LHR last year the MLG was pushed through the wing but the wing structure itself did not fail.
Marcus