I noticed that too but reckoned it was the final death throes of the Orc. You can't expect Boromir to kill all those Orcs cleanly, some of them had to have had crushing injuries which would disable them for fighting but take a while to kill them!
I agree, I think that the orc--orcs were merely wounded and not entirely dead. Think about it- it's really very difficult to kill a creature quickly with just one blow unless a warior stabs it in just the right place to hit its vitals. The Orc was probably in extreme pain and put his head back down so the world would stop spinning and so he wouldn't pass out... Ok now I'm just making stuff up. But it makes sense doesn't it?