Because of train tracks. It wasn't even that I was trying to go over them or anything. They were sunk into the road, but the road and tracks both curved (but in different directions), so that as I went along the curve of the road, the tracks ate my back tire, which threw me to the ground.
It was very very weird, and more than a little scary, and I left some skin behind and some gravel came home with me, but I am all ok. Sore, and I will be purple, but ok.
I paneled a comic about it at dinner, so that's happening.