Flame shot out of Galvanized Pipe Handrail when drilled

A. After having galvanized thousands of tonnes of handrail pipe for many customers, have never heard of this before. For a flame there needs to be three things.

Fuel (presumably a gas in this case?)
Oxygen (air)
Ignition (your drill bit red hot?)

The air and possibly the drill are explainable, but what was the fuel?

What colour was the flame? That can be a clue to fuel type. For example a colourless almost invisible flame might be hydrogen? A sooty black flame might suggest acetylene ... and so on.

There isn't an obvious (to me) mechanism for producing a combustible gas inside a galvanized pipe by accident. If you put a strong acid in there (say hydrochloric) then the coating (zinc) would produce hydrogen. But you'd surely know if you added acid to the pipe?

A. Hydrogen build up inside the sealed galvanised handrail. Ignites when hole is drilled due to hydrogen & oxygen mix requiring very low input of energy to ignite (e.g. slight spark or perhaps the heat from the drilling)