If it happens over and over, but not instantly, I would wire up an ammeter via the fuse, and check and see how much current is actually being drawn. Then you can start tracking down where the draw is, if it is excessive. 7.5 amp while one of the lower fuse sizes, is still about 100 watts, so it should be something that would be easy to find, if it isn't a random short to ground. Check the liftgate for worn/rubbing wires.