Excessive leak cause runaway? why?

Why would excessive leak cause the autopap to 'runaway'? Wouldn't just the opposite happen when there is no resistance:inthe nose and out the mouth? using pillows of course.
Forgot to tape mouth.

Not sure what you mean by "runaway". Any auto uses backpressure on the air hose as input to its pressure calculation. If the mask is leaking too much it can't sort out the leaks from your breathing.

Sometimes I move in the middle of the night and dislodge my mask. When it finally wakes me up I feel like it's trying to blow my head off. I often need to stop the machine and start it again to get the airflow down to where I can seal the mask again.

Because it can't hear you breath and it trying in vein to get air into you. Jim

What it really is doing is increasing the leak factor.