I wonder if soda lime glass `ages`, this is not the first time I have seen this?, UK collectors call this "ring off".

Tragically seems to be a problem with Thorn/Atlas/BLI tubes here. First time I've seen this happen on a tube across the pond tho.

Generally it's because of stresses in the glass. I think as collectors who keep tubes of all ages we see it a lot more, within the normal tube lifetime I reckon it's a tiny percentage that suffer this. After 10, 20, 30 or more years though it's going to be a far greater number. There are always going to be stresses at the ends (the cathode mount/stem is a separate piece of glass, possibly a different type?) and the physical connection to the end caps will generate stresses as the metal will have a very different thermal expansion rate to the glass. I agree though, the tendency for it to be the most treasured tubes to do this is downright maddening!

@LinearSLI-H:
'ring off' seems like a good name for it to me.

@Tom:
I rememer seeing a pic posted here of either a Thorn or Atlas with the end gone
I've had this happen to atleast 2 others (in those cases the end came fully off), One I know was just a 'nothing special' lamp, either GE or Philips, I can't remember now. The other I don't remember anything, other than being annoyed about it when I found it.
...oh and then there was the time I "finished off" an EOL F8T5 on a VHO ballast, after frying it I put it in the to be recycled pile...then a couple weeks later I happened to notice that both ends had dropped off ROFL
(I don't count that one since 1.5Amps through a little tube like that is some good abuse .lol. )

Yeah that'd make sense Zelandeth, I wonder if it starts with a minor flaw in the glass?

This tube is actually only a few years old, but they were uncommon to begin with, and now no longer made due to these stupid lamp regulations/bans.