Anyones fire ext go off from a hot alternator?

We were cruising along, drawing ALOT of current from the alternator, 400 watts worth of lighting, 1250 wats worth of stereo and the boat lights. Saw batt voltage getting a bit under my safe zone and turned off the 400 watts worth of lighting and about 2 minutes later the fire extinguisher in the engine compartment fired off?? Now it is only about 5 inches from the alternator and im sure it was warm but should it have fired just being warm (maybe hot??)?? Alternator is still working fine. Just need to replace/recharge CO2 extinguisher. Ill have to say im way glad it was not dry chem all over my engine compartment, that would tend to suc