Howdy sir molometer! well sir, since Hermann never mentioned this kind of treatment of any enemies before this, I think it was just because they had cut off the legs of their fellow warrior. They were so enraged by the cannibalism and cutting off
their friend's legs that they wanted to be extra cruel to them.

If it wasn't for that fact and since they hadn't treated other enemies this way then it would look like a ritualist practise like you said but that's a very good point that you make!