Clocks of the 'Vienna Regulator' type are right devils to set up; it's not for nothing that they have a marked swing indicator on the backboard; and they won't usually run at all unless perfectly vertical. This one started on its own, which suggests that the train that operates the chime had stuck and some minor change freed it. That could be a slight knock or almost anything. Once the mechanism was freed the weight dropped again, unhindered, and the clock chimed. Presumably it didn't 'go' by the clock itself starting; it needs the pendulum to swing for that to happen.
Chances are that the clock needs cleaning by somebody competent, who may remove oil put there by some well-meaning person in the past; clocks need no, or microscopic amounts of, very thin oil; and that the mechanism is sound and needing no work. Mind, then you have got to get the little B set up to run when it's on the wall!