My guess is between the cuts of video there was a note dropped with instructions to the POW. Like "follow the drone. leave all weapons behind. when you get to the stopping point, remove all body armor and helmet, lie down and wait for verbal instructions". And I'm guessing he got halfway to where he was supposed to, and realized he still had a grenade. By throwing it back the way he came it was a clear indication he was getting rid of it, what it was, and that he was trying to follow the instructions even if he made a small error.

All of this is my guesses.