^^^^ This. I had the same happen to me yesterday! The clip is like an oblong socket under the top edge of the visible bumper. A small impact to the centrer of the bumper partially breaks the clip and the top part of the bumper is pushed up slightly by the broken clip and the touches the bottom of the lower tailgate.

It's an easy fix, half open the tailgate and get your fingers in and snap off the offending clip. Close the tailgate and put a heavy weight on the top centre of the bumper and leave car in the sun for a few hours and it will be flat again. Simples!