How so? What makes rinse buckets such a flood hazard? (newbie question)

Several reasons, here are some:
1. This may sound crazy, but floods are much more likely at the surface than at depth, the deeper you go, the better the o-ring seal. Rinse tank = surface.
2. Rinse tanks are usually packed with cameras which bump into each other. That can displace ports, open latches, etc.
3. A lot of things happen between the time when you hand your camera to the dive master and when you actually put it in the rinse tank. It is possible that ports get bumped, latches open, etc, but since you just finished a dive most people won't check the seal again before dumping it into the rinse tank.
I flooded the battery compartment of a strobe once, a Nikon SB105. Because of the rush to get in the water a forgot to check if the sealing o-ring was in place, and guess what, it wasn't. I saw at least two cameras (not mine) flooded in rinse buckets. Unfortunately, no photos...