What could be causing the main breaker to continually trip?
This is a classic problem. The breaker connection to the buss is compromised/arcing/burned/etc. The fact that the breaker is getting hot and tripping under a load well below it's rating is clear evidence. This fact also says it is past the time when it should have been replaced. 

Tell your "maintenance man" to get a real electrician in there NOW! This job is well beyond the scope of a maintenance man.
It is almost a certainty that the enclosure/guts will need to be replaced along with the breaker itself.

I have photos of this exact situation from some of the ones I have done.