May be a rectifier diode from a full wave bridge failed in a high power dc charger, or other equipment and a dc riple ended up on the mains. Hence the braker had to interrupt dc and a arc remained for longer than expected?
The product of rotation, excitation and flux produces electricty.
Without any other information, that top breaker looks fried and the bottom one looks ok. Ditto the breaker tabs in the box. Perhaps a high resistance developed in the top breaker. I've had fuses develop a high resistance without going open, the same could probably occur with breakers - even Square D.