Interesting behaviour on Saddleback Butterfly fish

[howdy20012002]

hello all
I just recently brought in a few saddleback butterfly fish and have noticed a very interesting behaviour.
they hover over hermit crabs and rub up against them.
they do this for a few seconds at a time.
I have all noticed all 3 showing this behaviour in 3 separate tanks.
the other butterfly don't show this behaviour yet all the saddlebacks do.
any clues what this is about?
I was thinking something liking a symbiotic behaviour somehow where a crab cleans them??? i dunno if there is a crab like that or not.
that is what this behaviour sorta of reminds me of.
anyone have any experience with this?
thanks for the input in advance
Neal