What would cause this to happen to a pre amp tube?

[shortfuse]

You may not be able to see it but the top interior of the tube is white and chalky.  I know this happens when air gets in.  The cut around the tube is not sharp at all like it was broken and is a perfect separation around the tube.
This tube was in V2 before I started the mods I did on the 1959 plexi the amp only had about 10 hours on it before my friend brought it over.  Also how would it play like this?

[cobalt]

Hello,

Bad glass is all I can think of. Some times all it takes is a small scratch to weaken glass. The break is also in the "coolest" part of the tube so temperature differential may be part of the problem. Is that one of China's finest?

weirddave

[thermion]

The tube would not operate like that. I once had a power tube separate in this way, breaking nowhere near the seal and almost looks scored. Not common but not impossible either. FWIW, my bottle that did this was a russian 6550. I chalked it up a manufacturing imperfection.

[tubeswell]

One way or another it lost its vacuum - could be a leak around one of the pins