Mine do that too. The last one that did that wound up leaving its tube completely , shed its crown and has started growing another one.
my advise..
Leave it alone for a while. If it dies, any crabs or scavengers you have will get him.
If you find him out of his tube, just gently pick him up and put him somehwere out of the way, cover him up with a little sand.
thats what I did and he seems to being doing OK now.
I hope I was of some help to you.