What could cause a handful of dead phosphors seemingly overnight? RCA MM36110

[RCATUBES]

Hello, I've got an RCA MM36110 TV and seemingly overnight it developed a small vertical row of dead phosphors (maybe 1/4" to 3/8"). I've not been displaying anything that would have caused burn-in in that specific spot, at least not that I'm aware of.

Sometimes on startup there is a brief vertical flash in the center of the screen, but it's an inch or two ABOVE these burnt-out phosphors so I doubt that caused it.

I'm assuming at this point that its not fixable, I'm just very curious what might have caused such a thing to happen.

Image here: https://imgur.com/a/VKnfOSA

It's very small, but dead in the middle of the screen and extremely noticeable.