It is gravity when you are overhead.
Well, yes and no. If you were going a steady speed (which you don't) then with a mass that's going around a 62' radius of curvature* will pull about 2.8 g's. That leaves 1.8 over for line tension at the top of the circle. You'd need to slow down by about 66% (from roughly 51 mph down to a bit under 31 mph) to get to zero line tension -- although you'd probably be getting a wee bit concerned** before things got to that point, particularly in a stiff wind.
* This sounds odd because I'm being precise in my language -- just figure "rock on a 62' string" if it makes no sense to you.
** Mike Haverly fell in the contest this weekend in Classic, because a bad motor run made his Chizzler go slack on the lines, and losing his balance when he was trying to keep line tension. He did a very good job of not crashing while lying on his back and flying his plane. In spite of a reputation for graciousness, he was very gruff with me when I complimented him for having the foresight to fall on his a** to save his plane. Go figure. I had a similar slack-line event in Expert, for similar reasons, but I was able to just let it fly itself out until it hit the lines on its own.