What would make one of the hot legs on the main breaker burn and get brittle?
What happened to burn the leg was some combination of two things:


You were pulling excessive amperage through the leg without the breaker tripping to protect the circuit from the overload, resulting in overheating of the overloaded leg.
The leg was connected to the breaker lug poorly/incompletely, causing a high resistance which overheated under normal loads.


For the first, I'd use a clampmeter to diagnose the overload situation.  For the second, I'd check for a sagging voltage on that leg (as testing resistance from lug to wire would require the utility to pull the meter).