Why does the outside of liquefied gas cans freeze on the bottom while being used?

We have these 45kg propane liquefied gas cans at work, and while they burn their bottom freezes (i mean really, it looks like a dirty icebox) and I can't seem to figure out why. I'd go with PV=nRT (V constant, P variable ---> T drops because n isn't dropping fast enough), but that doesn't explain why it's only the bottom that's extremely cold. Anyone knows?