> fire around the rear flap hinges
I believe it's pretty hard to have a fire at that altitude. You need a leak of both methane and oxygen, and an ignition source.
I wonder if perhaps one of the engines split open and the exhaust wasn't going into the engine bell?
I mean blowing liquid oxygen on something with a hot heat source beside it typically turns things to fuel you wouldn't expect. Like metal.
At atmospheric pressure, yes.
But up at 140km altitude, the pressure is so low that I don't think even pure oxygen would lead to combustion.