I think I discovered one reason they may separate: snails. I had mine off overnight and, as soon as I turned them back on this morning, one of them separated. I found a large snail in the wet side of the one that separated; obviously, it got in there while the VTs were off, then blocked the prop from spinning when they came back on. As soon as the dry side rotated enough to repel instead of attract, pop! off came the wet side. My suspscion is that this may happen in some/most of the cases of separation, and the snail leaves when the wet side is sitting on the bottom, thus leaving no evidence of what caused the separation.