I know everybody seems to think that they are at the begining of a shed, but think about the other possibility. Thermal burns. You have an unregulated heat pad covered by some type of wood and more substrate? How and where are you measuring the temps. If the substrate is too thick, it will act like an insulator and the heat will build up under it, and when the snakes move the substrate and touch the glass, they can get burned. You might want to invest in a thermostat. Do you keep these snakes together all of the time or only for breeding?