Unusual Tube Death

This is from my friend JC's tube amp. He is the guy who built my Jubilee, SnailTrac bass horns, and has also sent me cool horn stuff to play around with. He said he was listening to music when there was a horrible noise and a fuse blew. I've never seen this type of tube failure before. Which went first the glass or the fuse? How could this happen? Opinions welcome.