I deliberately said "higher powers" (unspecified) because this is another one
of those moments in which Tolkien leaves the door open to a divine calling
without having a cartoon God hand pointing down from the heavens (a la Monty
Python). Circumstantially, it could very well be Manwe because we know
his eagles are waiting in the wings and the winds are his domain. Eru
himself now could be tinkering with the design as he did with bringing Gandalf
back.
Tolkien does not give a clear delineation of what kind of intervention may be
going on here, and skeptics could still read the calling psychologically.
That's what's so powerful about his writing: he embeds the workings of
the higher powers into the fabric of the story so seamlessly that it does not
look like an overt deus ex machina even though at the deepest level
there is much divine machination going on behind the scenes. It is that
nudge to keep them on track and helping them subtly at the moment they most
need
help.
"'Never laugh at live dragons,
Bilbo you fool!' he said to himself,
and it became a favourite saying of
his later, and passed into a proverb."
--The Hobbit
