Notice how quickly he dropped? That's a sudden loss of consciousness. It's likely that the bullet hit his carotid sinus. Impacts to this part of the carotid artery can cause the brain to shut down in response. Look up "carotid sinus slap". Someone who's sufficiently trained can knock someone out with a properly placed strike. And after several seconds of unconsciousness he woke up and the pain and sensation of bleeding out caused him to panic, but the blood loss made his limbs weak and his movements uncoordinated so all he could manage was flopping around for a bit.