What is it called when you coast on a bike, you stay on, but when it stops, you fall off?

Dynamic balance.

Inertia. When the bike stops, the body keeps moving.

The bike in motion maintains dynamic stability, if steered correctly.

Balance is much harder to maintain without the stability afforded by constant forward motion.

loss of momentum balance

the bike is gyroscopically self stabilizing, as long as the wheels are moving.

i guess you could call it that

when the wheels stop turning the gyroscopic effect disappears. and you fall over.