What exactly makes this fact outside of a few shooters thinking they notice such a phenomenon? I could say since my groups exponentially grow as distance increases that bullets not only travel out in a funnel, they intentionally try to miss what I'm aiming at. Does that make my statement any more fact than yours? Its what happens when I shoot. Must be fact. Right? Its a theory and that's all. Until there is hard data beyond a bunch of guys shooting in the wind it will be nothing more than a theory.Whatever the phenomenon referred to as "going to sleep" is, it is fact.
As for what is happening, well, there are a thousand variables to pick from. I would start by removing them one at a time if you really want to get to the bottom of what is happening.