wet medium in tank for NM Kingsnake

I have had this rescue snake for several years and he has been doing great.  In the last 2 months the medium under his hide has been wet every few days. 
This is the third time I will be replacing the wet medium with dry. I can't get a vet appt for a few days because of the holidays and wondered if anyone has had this experience before and what is causing it and what to do.  THX