the way that i understand it, is that around most cars wind travels at a hgih rate of speed...if you ever see video of a car in wind tunnel testing, the smoke tucks over and around the car. as the wind sucks back in behind the car at highway speeds, it creates vacuum which is enough to cause some pull to the light rear end of a bike....you can even feel this in a car when you pass a semi, and it is especially bad if youre driving something like a motor home, with a large flat side, and you pass a semi at high speeds, because the two flat surfaces, pull each other together...

here is a very crude illustration...

[IMG]local://upfiles/4/8A44CF5DA2E1477FABC8D6417E715E50.jpg[/IMG]