How big is he? Because small or teeny crabs can molt alot, really fast. My teeny, George, has molted four times since August. His shortest one being less then a week, his longest being three weeks, and now he's a small!
Have you checked for needle feet? Or was he displaying molting signs before this happened? (ashy exo, excessive drinking/eating) He might not be molting at all, he might just like it underground! I know some people, me included, have crabs that almost never surface, but they aren't molting, they're just being little stinkers

Another possibility is that he's trying out different spots and different tunnels before he decides which one is the best to molt in
