As it's a -400 ("bathtub" apu containment) and the door is closed the only thing the fire department would be putting out is a tailpipe fire. Could be someone wasn't told the thing to do is to keep cranking in that case.
It's also possible that the starter failed after ignition and there was a hung start which is a bit too common in the classics. Not really common but it happens often enough. The pump is putting out more fuel than the combustion chamber is capable of handling so the excess burns off in the tailpipe. Makes the EGT go scarily high sometimes.
One other scenerio is one that happened to me in
PHX a couple of years ago. I was starting the APU on a -300 after someone had shut the thing down with the battery switch. There was a bit of fuel in the tail pipe and it lit off and was burning slowly as the APU wasn't on speed yet. A passing fire vehicle stopped after the driver saw it and filled my tailpipe with Dry Chemical without asking anyone. When he came to the cockpit to see if everyone was all right and get his thanks I was just getting out to see what the H... was wrong and my boss was running up behind the fireman with blood in his eye.
We all had a good shout and the fireman bought a few beers and promised to ask next time before he tried to ruin a perfectly good APU.
Aviation, it's a vocation and an experience, not just a job...
