Happijac failed while driving down the interstate!

There is a version of Happijac that has a drop leg - basically a way of releasing the motor and brake, which allows the ball screw to unwind under its own weight. A lever engages the brake. What model number Happijac do you have?

On the normal one, there are two mechanisms that keep the jack up: the gear motor and an internal brake. Both would have to fail for the jack to unwind by itself. Either, if working properly, will prevent it.