You are just adding gravity without any limit to the y velocity. That might have something to do with it.
In truth gravity is a acceleration towards a mass, and in our planets case if you were to jump off of a plane when skydiving you would experience this acceleration to a limit.
Since there's the atmosphere cushioning you and offering friction you would only accelerate up to what is known as the terminal velocity. A point past which you wouldn't accelerate any further.
The ever increasing gravity induced acceleration would also likely mean ever increasing friction against any floor, no?