Squib load then a normal follow up shot, the bulge in the barrel is the giveaway. A double load would have grenaded the cylinder.
That's what I'd suspect, though I've seen a double load literally turn the bullet sideways in the barrel and blow the whole side out of the revolver, leaving the bullet in the remainder of the barrel too. It was a Ruger RedHawk, in .44 Magnum. The cylinder looked a little worse than the one pictured.
Can't get my head around some double charge accidents. It's tough to put a bunch more powder in a .44.
The video below is in essence the same effect as a squib.