Mine typically do this if I put something too close to their anemone (I have a 4-5 in. area around my nem rock that my clowns refuse to let me put corals XD) . If they have a favorite spot where they hang out they may also do this to keep the area clear
Are they hosting something close to that spot? Have they laid a clutch yet maybe in that immediate area?
It was the male that did it, I think he might just be protecting the pair's area/nem/eggs. Look under the rock and see if they laid a clutch
They are hosting my torch and hammer since there are no Nems in my display tank. I ended up moving the coral to another place until I give them to to their previous owner.