I don't think I've been on a bus here yet that hasnt' been driven too fast! Like someone else mentioned, they drive inappropriately fast. Back in the UK, there are roundabouts with 60mph limits, but that doesn't mean you're supposed to go 60mph around them! And quite often bus drivers here (and taxis for that matter), pull away with massive acceleration, only to sit at a red light 5 seconds later (fuel consumption anyone?)
I'd say it was fairly easy to tip a bus, just speed around a roundabout with a lot of load on the upper deck, and if you really want to make sure, clip a curb. IMO, 99% likely to be driver error, 1% likely to be bus company/triad conspiracy. What kind of mechanical error would cause a bus to randomly flip over anyway??