When an old lath and plaster ceiling drops its plaster on you, look up between the laths. If there are no remnants of plaster between them, or very few, then what happened is that the original plaster just wasn't pushed up hard enough. It's supposed to go up between the laths, and slump outward to make keys that lock it in place. But initially, it'll just stick by surface tension and friction against the sides of the gaps. If the old plaster didn't have good keys, after a while the laths will shrink a little and it will fall. It could take 10 years, it could take 80 years, you never know until it happens.