They have to find a way to turn . In some cases, this may occur when there is too much flow which causes them to move. Their foot is made of muscle fibers and tentacles cells.
I was about to ask if my BTA is in trouble. If I poke it I can manage to get it to shrink and it's back to normal, but the next day... it's twisted again. Definitely it's due to the flow. Its "mother" lives a few centimeters away but stays in place only moving a bit when it's directly fed (which is not often). I included a pic of 30 mins ago...
It started to twist even more so I poked it again and again and now it's finally seem to be moving to where there's not much flow (and no poking!), it's a little shrunk now but that's fine