It is all about center of gravity, and the tipping moment of the train. The suspension plays a role, the condition of the track, etc. The tipping is caused by external forces (wind, rain) and by self generated forces (harmonic swing, vibration, uneven track, passenger or animal movement, unrestrained liquid flow in tank cars). The suspension can accentuate the harmonic swing of a vehicle, and actually increase the swing until the vehicle tips over. There is then of course a minimum track gauge, and then progressive factors of safety by making the gauge wider.
The San Francisco BART was built to broad gauge because of concerns that the light equipment would not be safe in high winds on the viaducts (and at high speeds). Washington DC metro, although almost identical, uses standard gauge, because most of it is underground or at ground level.
There are some conspiracy theories that BART was built to broad gauge to create an expensive "monopoly", but BART really does have very long stretches of raised viaduct close to ocean winds.
The outside frame DRGW power offers a very low center of gravity.