I have noticed that they are more likely to detach if they do not like where you have put them. The good news is that they will let the wind (or in this case water) carry them to somewhere they like better. It's not necessarily a sign that they are unhealthy -- it simply means they are looking for a better spot. Good luck at trying to pin them down. Its not easy. I would make sure your tank conditions are suitable for growing mushrooms.
I have several that picked up and moved, and I'm not sure where they walked off to. I imagine that when I tear down the tank, I'm going to find them all behind a rock where they've been conspiring,