It is normal behavior. They are ambush predators and their natural environment is marshy and allows for that. The only thing I would be concerned about them burrowing in aspen is how dry it is - and being in that much surface contact (when burrowed) well you get my point.

Unless conditions demand otherwise I keep mine on newspaper and have a cat litter box filled with moist sphagnum they can lay around in or burrow in - they use it with great frequency. We got a cold snap not too long ago and I had to crank up the heaters down here which resulted in the room humidity dropping sharply. At that point I just kept them on full sphagnum. The nice thing about sphagnum is the high acidity content makes it harder for mold/mildew to get a foothold (possible - just not prone) This is not a prescription - just an observation based on the 3 we have. It's early in the day yet - other blood owners will chip in I am sure!