Today while on the way home from work on the bus, a fluorescent fixture that illuminates the inside of the bus, had stopped working right above my head. It didn't just fail, however. It got very dull, and a dark space grew from one electrode right up to near the other end of the tube, with a tiny portion of one end of the tube, it isn't the heater as the dark space shinked for a little while, then back again, then the whole tube lit up, albeit dimly.

Then some striations appeared in the tube, but they were going really fast. Why did it do this? I know that tubes keep flickering because of a supply having undervoltage or a bad tube, but not like this! 

Oh yeah, it was a T5 13w fixture, running on an inverter ballast from a 12v DC battery supply as it was a vehicle. During the period the light was on, the ends of the tube were very blackened, you can see the blackening and striations through the diffuser.

That sounds like just a dying tube especially since it is DC my old RV when the fluorescent tubes went bad they did the same thing

I wonder if it's one of those "cold cathode" fixtures. I've never seen cold cathode burn out...seems interesting.  If both ends were blackened, my guess is that the inverter converts the DC to AC. I've got to get an inverter one of these days.