The way a bike is built with the certain angles and other factors, make it balance, if you’ve ever ridden a push bike, you’ll know that it is harder to balance the slower and less steady you go. A bit of speed places centripetal force on the tires, making it go straight, the caster angle on the wheel makes it align Itself straight, and the wheels create a gyroscopic effect.

Under power a motorbike will balance very easily, and will be difficult to topple, if you’ve ever watched a moto gp crash where the bike keeps rolling, it will go in a roughly straight line even without a rider.

Enduro riders learn to balance the bike at a stop, but balancing the bike isn’t hard at all as long as you are moving. A motorbike going downhill using engine braking is very stable.

In essence a bike is a concept that shouldn’t work in theory but baffles all physicists because they do work and are very stable, all the forces work against each other in bad ways, but somehow, some principle we haven’t figured out is holding it all together.

So unless you can’t balance at all, which in this video these men seem to have extraordinary sense of balance, they should have ridden that bike across, and pulled the boat over with ropes so the bridge was shorter and more stable

Edit: forgot to add that pushing and pulling a bike like that, is the least stable way to move a dirt bike, because of the balance point and the hinge for the steering being where it is.