Thats just something they do when they loose their footing sometimes. No way to prevent it, you could add some floaters or something for them to grab onto when they hit the surface to help relieve the angel attacks.
Went to feed fish last night and the Ramshorn was on the bottom doing his thing. Interesting to see how far they come out of their shell while scavenging. My Nerite hardly moves at all. I have a big blue shelled MS and he enjoys being half in the water and half out. You think my water conditions are not right for him?