The truth is, that hasn't really been figured out. I have seen Paul's engagement in another thread a while back where he touched on it, and said that it could arise from a shoe being stored too long in a shoe box laying on it's side, or not being kept with shoe trees. It was strictly conjecture, however.
I have asked questions about it, and the only responses I seem to get are that the bowing is present when people buy the shoes, rather than something that develops over time. I have also heard of a person or two that figured out that changing sizes fixed the problem. The issue really is that it is speculation regarding what is causing the problem. I'm not saying it isn't a defect, but there is anecdotal evidence to suggest that it isn't.