The only thing I could suggest is put new pantyhose over the filter intake if you're worried about it. Sometimes bettas bite cuz of pent-up agression, maybe you could use a mirror exercise to give them something else to have a fit at. but not for longer than a minute, no need to stress them out.
My VT Phy is a tail-biter, and it's been non-stop no matter what I do. So I just keep the water clean so it doesnt get infected and recently put him through some aq salt when i thought he was getting bad. I've been told Stress Coat can also help in keeping infection out for tail-biters, since they're initially re-opening sores that are trying to heal.
As for them all doing it at once...Iunno. Fish pheremone? xD