"Why would someone shit on an escalator?" seems absurd enough to be the setup for a joke or a Zen koan, but I'm going to ask it anyway. Why would someone shit on an escalator?
BART stations close shortly after midnight, and they are closed by locking gates at the bottom of the stairs and escalators. This means the bottom of the escalator is relatively private, and if you shit on the escalator itself the shit "disappears" when the escalator is turned on.