Clinton's support goes up and down, but Trump's had a hard ceiling so far that he's yet to really break through. I doubt he's getting a lot of undecideds when November comes around.
But Clinton losing support seems to be the main reason it's tightened.
Also, the media is going to try to further tear Hillary down so they can have their "horse race". I'd expect this in any election from here on out in which there is a clear frontrunner.
This. Let me remind you folks that Trump has not hit 45% in
any 3/4-way national poll that wasn't CVOTER, Gravis, or LA Times. In fact, his highest point in non-junk polls is 44%, which was in a CNN/ORC poll conducted immediately after the RNC.
I think it's laughable that some people (Democrats included) think the race has tightened significantly based on some landline-only Emerson polls.