Damaged SEC

I found this at yesterday’s inspection. This is an older square D 125 amp split bus panel. The SEC on the left side overheated badly at one point. I am curious how this might have ocurred. A short at one of the branch cicuits would have tripped a breaker. The branch circuit wiring looked original and in reasonably good shape, aside from the bare wires, double tapped neutrals, and the missing strain relief.

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Appears that the lug was not tight enough, or loosened over time causing heat.