this is my reply. 
Because it is simulation, so kind of quantized time and impulses, and simulation gives funny glitches sometimes. Even smallest assymetrical rotation usually increases over time, and without stabilizing code it just flips vehicle over. And that is on flat horizontal surface, he want to have that track in 3d. Vehicles will do random physics glitches all the time, preventing that is hardest part of coding such game.
Sorry for that edit, its to easy to click wrong button.