I'm quite surprised by that to be honest. In the past the only 3rd party that would siphon votes from the Democrats were the Green party, while the Libertarians and that brief run of the Reform Party under Ross Perot would traditionally pull voters that leaned right.

Given the margin for error and that it's still early I imagine the Libertarians will eventually drag more from the Republicans than the Democrats, but that's a rather surprising result to say the least (perhaps some right-leaning voters feel they already have their anti-establishment candidate in Trump?). At this point it seems as if it's actually likely that Johnson would poll high enough to warrant a position in the debates should Trump and Clinton wind up being the nominees.

Personally, I think the threshold should be lowered to 10%... 15% just seems a tad high and while the CPD has decent reasons listed for coming up with that figure one has to believe the influence of Ralph Nader and Ross Perot on the outcome of the elections had neither Democrats nor Republicans very pleased.

I think it is mostly Bernie Sanders supporters not content with Hillary, and they think Johnson is a good non-establishment alternative who is honest, even if he isn't a leftist (most liberals seem more strongly liberal than they are left-wing so Johnson fits that bill.) 

A lot of people are not content with Trump either. If Gary continues in the polls at 10%, I can see people pushing him up to 15% for the debates. I hope it happens. Johnson isn't the best libertarian (and is V.P is atrocious), but I he is good enough in my opinion, and being a moderate libertarian, but still identifiably libertarian, can help things more than hurt I guess. The issues that he is wrong on most people are wrong on (freedom of association), so that is okay I guess. 

Here's to hoping. :) Lot's of people want an alternative to Trump and Clinton, and a Johnson presidency while having very low chances of winning, can put Libertarians on the map for future elections.