That was one of the early theories but it hasn?t stood the test of time. I?m not one for subjective photographic interpretation, so I?ll keep my hat out of the ring as for specifics as I don?t see the issue as being especially relevant. If the debris flew backwards but only so far as the trunk, that?s no smoking gun for conspiracy.
Unfortunately, at one time, even I thought she went "reaching" for or "grabbed" a piece of head matter. It wasn't until I saw a stabilized, crystal clear print of the film that I realized what happened with her right hand as soon as it met the surface of the trunk lid, then saw her land on her left elbow/forearm hard enough to move her entire upper body. Conspiracy theorists constantly use the actions of Mrs. Kennedy to claim conspiracy.