There are several things that can go bad with tubes. They can become gassy,
they can have element shorts, etc. so it is hard to know exactly what happened
there.

What probably happened is that one tube became weaker and/or failed. The other
tube assumed the load and was stressed beyond its ratings. The total load was
probably too much for the power supply and the fuse blew.

Just a guess. Without inspecting/testing the tubes it is hard to know.

73s,

Evan