Not necessarily. Digging can be a sign of molting but that's not a hard and fast rule. Some hermit crabs will dig down for a self induced time out. They could be relaxing, rejuvenating or possibly hibernating. Some crabs are known for digging just for fun. Ecuadorians in particular are diggers. After you get to know your hermit crab a bit, you will know if its a digger by nature or prepping for a molt.