It looks like heat marks, more than wear marks. Maybe that ballast you have overdrives the tubes severely? Or the cathode heating is too severely intense, causing severe heat in the cathodes, which can cause blackening on the phosphor itself within minutes.

I've done experiments with severe overdrive on larger T8 tubes and severe end blackening can happen within 10 minutes at twice the rated power. It looks like the tubes I've tested with have been running for years and years even though they haven't!

For something that appeared in seconds but then does not continue growing, i would guess some foreign substance on the electrode, which then either is flash-evaporated, or (if it is a dust of some form) deposited from it electrostatically when power is applied. I have no explanation what it might be and from where it got there

What i do wonder is - In FL tube manufacturing, the cathodes are heated quite brightly for emitter activation, and tubes are tested for having vacuum by lighting up for a second - and i guess there is nothing more than applying high voltage in instant-start way. Whatever the blackening is, why it does not appear already there and then ?