When I first put my crabs in my tank, with extra shells, I swear they changed shells once or twice a week for several months. Sometimes they'd keep the new shell, sometimes they'd switch back to the old one. My take of the whole comical situation was that crabs also fall victim to the "grass is always greener" mentality we humans do!
Eventually, they all settled down and found shells they liked. In fact, of the four crabs I still have, they haven't changed shells for the last 6 months... at least.
Soooo... if what I experienced was "normal", I'd say your crabs are just being crabs and they'll eventually settle down. As long as they have extra shells, you should be OK. I don't think they're looking for anything in particular... they're just lookin'!