I'm pretty sure his original comments on his crash stated that it was a single vehicle accident and that he lost control and spun out, hit a pole or fence or something like that, and flipped. I don't know if anyone else caused him to have to swerve or anything that may have helped to cause it or not. Details were posted somewhere but so far I haven't found it either.
EDIT: Maybe he'll chime in... looks like he may have been broadsided after all. Or he couldn't help broadsiding something on the side of the road, and it initiated the roll?