Is the grounded conductor aluminum? I found a problem once with a feeder where the conductor degraded inside the insulation.
We tore into a wall and found a bubbled spot where the conductor was overheating. Good idea to shut it down and check everything. An overheated connection could break loose
and BOOM! Unrelated but check for missing buss supports also. I just opened an older Federal Pacific pull section to find 2 of the critical main buss supports missing.