Crytek changed the behavior of NPCs when in this super punch mode because it often times killed you in the process. Before patch 1.2, hitting an NPC with a high strength mode caused them to fly into space, but it also sometimes caused them to ricoche back and cause instant death splash damage on you. This happened a lot when you punch an NPC with a wall or object directly behind them. Because the strength of the punch is porportional to it's splash damage to you or any other NPC.
Was kinda funny seeing a guy fly into space with one punch though.