Was it a hot day in combination with you using several 12 volt
lights and/or 12v
systems. If the answer is yes, that is the way the converters are supposed to function. A thermal switch will turn the fan on to cool it down.
If it was a cold day and you were not using any 12V
systems, you may have a shorted battery that is overheating to the point of causing a dangerous situation that should be looked at immediately.