Maybe the tree stump was all that was left of an Entwife, with just enough consciousness to realize that these were good guys, since they had Elven rope, so the stump released the rope?
(No, this is not my serious standpoint; I'm one of those people who throw a punch to get a fight going, then step back and watch the others get hurt! 
)
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'Mercy!' cried Gandalf. 'If the giving of information is to be the cure of your inquisitiveness, I shall spend all the rest of my days in answering you. What more do you want to know?' 'The whole history of Middle-earth..
.'