So I
tried writing an article on the topic, but even after finishing it I am no smarter than I was. So why do you think Watcher went for
Frodo? Was it because he was the closest tasty morsel, or because
the Watcher was drawn in by the malignant energy of
the One Ring?
I think the idea is that some agent or other dammed the river and caused a flood to join with the underparts of the
Misty Mountains, which enabled the Watcher to rise to the surface and remain there. This was done to trap
Balin's expedition from the West so they could be attacked from
the East, as in the Book of Mazarbul: "we can't get out!".
So, this is speculation, but I think you need to look outside
the Ring story for an explanation. I don't think
Sauron knew or cared about Balin, but perhaps the
Balrog did. In that case the Watcher was to stop any movement in or out of
Moria. He might have wanted to go first for anyone who had already entered Moria,
Gandalf then Frodo I think, so Frodo would be the hindmost at the time the Watcher emerged. AIUI all the others were still outside and could be picked off or chased away later. There might have been other
Unnamed Things waiting to join in the fun too, especially if Gandalf had chosen the wrong path of the three inside Moria!
OK I didn't expect to write so much
