I don't know if this has anything to do with it, but I recall that there are old lava tubes like a vascular system all through this area and not terribly deep. They run fromMt Blanca (1st time I saw it I said, "now that's a volcano!") and from further east from Spanish Peaks and even further east down into New Mexico. Two place I've noticed activity here and that is in this area, but also up by Steamboat, which is on the northern border of Colorado but pretty close to due north. If you drew a line from Yellowstone to the volcanic area west of Santa Fe that I can't remember the name of without more coffee, it would be pretty straight.
Just stirring the pot,
Beth