The brakes can fail, any weight on the escalator then slams to the bottom. I'm talking about a failure, not simply turning the escalator off. Never attempt to walk down an escalator that's experienced a mechanical failure. It's dangerous.

https://youtu.be/VWLb7P8MCck (edit: wrong video initially, this is more what I'm talking about though I can't seem to find the specific video i'm thinking of of an escalator violently accelerating.)

Found it! https://youtu.be/3lt3gELY3pU

It's scary as fuck.

I've also heard (though I'm not sure how true it is) that the motor going out can make the escalator belt go slack allowing the steps to flop to the sides.