do you think one of the rocks shifted and fell, causing the crack?
Maybe be a little more careful about your set up, wiggle each rock with your hand before you let it be, to make sure its secure.
Dunno if you guys have opened up your ceiling, but its good to cut some holes in the damaged drywall and get some fans running to make sure the joists get dried out and avoid further damage like dry rot. Talk to your dad about that first though.