tBuOK is very hygroscopic. Its likely that's the source of your moisture if you've eliminated all other possibilities. If your bottle smells like tBuOH there's definitely a problem.
I've heard about someone starting a fire by doing the resublimation* procedure wrong, but they opened the flask while it was still white hot so what do you really expect. If you've got the equipment, read up on exactly what you're supposed to do, and don't do anything stupid/insane in the process it should probably be ok, but overall it sounds like kind of a pain. Otherwise, its not a really expensive reagent. I like to get a small amount a few days before I need it and not keep it around when I'm done.
On another note, I've heard that potassium neopentoxide is almost as good a base but easier to keep dry. Anyone else have any info?
*I've always thought it was really cool to have gaseous ions flying around. Who thought to resublimate that stuff in the first place?