I'm surprised it didn't kill him, maybe it was old and deteriorated explosive.
I'm kind of reluctant about this but I'll go ahead and indulge in some "war stories". Years and years ago there was a place built out of tires and railroad ties used for mout training called Regansberg. A group of soldiers stacked at a doorway and the lead guy threw a frag in. The grenade bounced out and the solders all dropped on their bellies, the grenade exploded on the concrete just a few feet in front of the first guy, none of them were injured at all. Later I would read about the same thing happening in somolia. A couple years later a group of soldiers were walking in a line abreast and a grenade exploded at one soldiers feet, he absorbed the brunt of the blast and was killed instantly. The soldiers standing 20-25 feet on both sides of him were both knocked unconsious for minutes. They both suffered minor shrapnel wounds, some permenant hearing and vision loss. But it could've been much worse had they been standing closer, or had that one guy's body not been there to absorb the explosion.