After the last shot, if the sniper had just sat straight up from his leaning forward shooting position, I believe that, from Brennan's position, he would have appeared to have stepped back and into the corner.
I don't believe the sniper was sitting on anything during the second and third shots, so his "sitting up" after the final shot makes no sense to me.
He was squatting or kneeling during those two shots, imho.
-- MWT
PS Uh oh. I see T.S. is still cuttin'-and-pastin' together an entertainin' "post" for us, here.
Been workin' on it for at least twenty minutes ...