Hermit crabs may abandon their shells for a variety of reasons. Uncomfortable shells, low or high temperatures and humidity levels, stress and molting are the most common explanations.
Cracked or dirty shells, or those with loose debris, such as sand, may cause the hermit crab to leave its shell due to itchiness or irritation. Crabs are also known to refuse to go back into shells they have outgrown. Humidity levels below 70 percent or above 80 percent may cause stressed crabs to vacate their shells, as well as temperatures below 75 degrees or above 85 degrees. While unusual, some climb out of their shell to molt on occasion.