Breaker was probably tripped and reset many, many, many-many-many times before this scenario.
Sounds like the frame failed to contain the arc (probably a bolted fault condition on the load side),
The AIC of the frame was highly exceeded,
The frame fell apart and caused an arc fault,
All of the above.
As pointed out by Don, find out why a breaker is tripping first. Then only after correcting the problem would the breaker be reset.
If it trips again, there are more problems to look for.
When 480 VAC Power Circuits are tripping, be REALLY careful when resetting breakers!!!
Find the fault(s) first!!!
Reset the breaker last!!! Do it only once!!!
Arc faults are very mean Animals!!! They are like Badgers:
Very belligerent, extremely fast, and most of all, very unforgiving!!