No actual proof is being offered as to the alleged shirt bunching. Saying it did is merely verbiage. And as usual, the visual aid shows the bullet exiting JFK through his sternum.
Since the jacket has been shown to have bunched up -- and the shirt had a similar pattern of displacement, as the bullet holes in the jacket and shirt demonstrate -- then the shirt must have been bunched up. The shirt being lighter material, it was probably bunched up near the shirt collar in a series of wrinkles. The jacket was of denser material and bunched up in a single bunch.
The sternum (which somewhat resembles a necktie) is clearly below the exit site shown in the line drawing.
LBJ made sure he stacked the WC. His ace-in-the-hole being Gerry Ford. Ford moved the location of JFK's back wound and was the FBI/Hoover's eyes and ears inside the WC. No job was too big or too small for this soon to be appointed President.
Ford thought the initial description (something like above the shoulder) was too vague (sounds to me like it could even be read as a missed shot). Ford convinced the other Commissioners to employ a description that better reflected what was in the autopsy report. Nothing was "moved." It's a little incidental thing that really captured the imagination of the conspiracy-minded.