Why can closing a circuit breaker into a short circuit result in an arc flash?
Depending on the supply, the prospective short circuit current could well be in the kilo-amps range, even if the breaker is only rated for tens of amps.

In such extreme cases, the breaker is simply required to break the current.  It may not survive the damage.  While it is breaking such a large current, there may be a substantial flash.

I wouldn't want to be the person holding the lever of a circuit breaker while it's in the process of breaking a 10kA fault.