Probably the Hurt report (yes, that's what it was really called, compiled by Dr. Hurt) back in the mid-80s. It's way out of date though, and apparently single-vehicle accidents have become more frequent. Still, a large percentage of car/bike accidents are a result of the failure of the car's driver to yield the right of way.
I do believe the Hurt report is where that stat is from and that there may be more single vehicle accidents now (bikes). What hasn't changed is that cager pullouts and left turns in front represent the majority of bike v. cager crashes. As for single bike accidents, I'd think getting into a corner too hot and riding under the influence are the biggies.