There are many reasons you might find a hermit crab out of its shell, one of them is when they molt.
One of our large hermit crab just completed molting and we caught him without a shell basking in the salt water dish. Apparently they are very dehydrated during this process so he has to soak up as much moisture as he can during this very delicate process. His limbs and body are extremely weak and for a brief period, he even lost all motor function and toppled to the side. But surely enough, he regained mobility eventually and crawled into his new shell. Check out the video to see him safely back into his new shell.