This seems to happen every election cycle.

Here is 538's graph of the poll average. As you noted, it shows a steady decrease in Clinton's odds over Trump following the end of the Olympics.

For comparison, here is RCP's poll average from 2012. Note that in late August, Obama's lead steadily diminished until Romney was actually leading temporarily in early September. And here is the 2008 graph. Note the same behavior; Obama's lead falls in late August, with a temporary McCain lead in early September.

The fact this seems to happen every cycle suggests that it's intentional. My guess would be that Obama's campaign team (who are largely running Clinton's 2016 campaign) have decided that there's no point campaigning much in these weeks, and instead may be focused on fundraising. A lot of people have noted that Clinton seems to have disappeared, e.g. Politico has had articles claiming that she's just running out the clock. I remember feeling something similar in 2012, it seemed like Obama wasn't campaigning much then. But then in the leadup to the debates, he became a lot more visible. So maybe we'll see the same thing from Clinton this year.