Probably CO2 - a reaction product of the Carbonate alkali in the developer and the acid stop bath. I can't recall offhand how dependent this is (or not) on the pH level, concentration of the stop bath etc. Not sure why it would happen sometimes and not others, but my understanding is only a small amount of CO2 is formed and that it quickly dissolves.

I doubt it has anything to do with TAF-1, which is probably just Benzotriazole or a similar compound.