I can hazard a guess, but that's what it is, conjecture, not facts. When you hit the road and started to slide, your clothing was also moving. It's possible the denim in your jeans abraded your skin in that area, and it's possible the jacket slid down far enough to expose skin. Without video evidence to the contrary it's hard to do more than speculate. I'm assuming you're recovered and replaced your safety gear.
Back in 1978 I was riding my XS650 Yamaha on HWY 18 just outside Manhattan, KS when a very old dude in a very large Buick Electra 225 pulled out in front of me as I was nearing Stag Hill Country Club and Golf Course at 65mph. I ended up in a ditch after flying over his hood and had broken bones and quire a bit of road rash. Both of my knees were bloody, but as I hit the ground on my back and slid down the ditch the only thing I could guess that caused my knee to scrape was going over my handlebars, which I broke off during flight takeoff, and probably with my knees caps. The front wheel was embedded in the engine. With everything happening so fast for the recipient at the launch point it's hard to recollect the exact sequence of events to any great degree.
