[bcarver]

sorry about your injuries, Hope it rehabs well.
Question: was a fresh primer on the weapon?
If you put the primer on first it could have been your fault.
but only if primer went off

I blow air down the barrel and it is safe as long as the gun is not loaded.
Shoot, cock gun, remove primer/cap, blow.
It does look dangerous but unless you load it and forget you did it would not be dangerous.
How could that be unsafe.
The extra air helps burn the embers. patches make this problem worse as threads might lodge in bore and smolder.
The ember theory seems most likely.
as you were pushing the ball down the barrel it may have pushed an ember down to the charge