Drywall falling from ceiling
First, you should let your landlord know ASAP, before the problem gets worse. The ceiling falling down is not your fault. Not reporting it and letting it continue to fall down would be.

Anyway, most of what you've mentioned is probably unrelated. Running a small humidifier is perfectly normal. Cockroaches are not heavy enough to pull down a ceiling (picturing that is pretty gross...). Loud music and tromping around (something all upstairs neighbors seem to enjoy) wouldn't put any stress on your ceiling beyond normal limits.

However, the water leak seems like the culprit -- the water may have damaged the drywall around the screws enough for the screws to pull through the drywall, causing the sheets to come loose. Or, the ceiling may have been installed poorly, with butt joints not fastened to joists and just taped together; such joints would have eventually failed anyway, but the water damage accelerated things.