538, as usual, has been pretty insightful about Trump recently. There's this piece, and this one as well: http://fivethirtyeight.com/datalab/two-good-reasons-not-to-take-the-donald-trump-surge-seriously/
A lot of the bumps in popularity right now are purely name recognition: People hear a list of names, and pick the one they recognize the most. As the election gets closer - and especially when debates happen - other names will become more popular. As khjuu mentioned, this same bump happened to other candidates, but goes away pretty quickly once the race gets serious (i.e. the primaries are finished).
And Republican voters are eating it up! http://www.washingtonpost.com/politics/poll-trump-surges-to-big-lead-in-gop-presidential-race/2015/07/20/efd2e0d0-2ef8-11e5-8f36-18d1d501920d_story.html