I vote for a failed bonnectiom where the breaker was bolted to the bus.
Take a close look at where the breaker was bolted to the bus. A bolted connection may not have been torqued sufficiently and may have started to get warm under a load. When the load is reduced the connections cools and the heats again under load. This repeating cycle loosens the connections further increasing the heating to the point where the connections starts to fail and begins to arc across itself. Increased arcing ionizes the air around the arc and involves an adjacent phase and then the neutral of ground that would be close by.
