I should have taken the picture while it was still bleeding.
Ultimately the cause was biking in sandals instead of my bike shoes. You read right. We were lazing around in beach chairs at 5:30p.m. and Steve suddenly said, "Want to go for a short bike ride before it gets dark?" It gets dark at 6. He was already halfway onto his bike. This was an unusual invitation and if I took time to change shoes (a) he'd be gone and (b) I'd be a geeky jock for needing bike shoes for a casual ride around the park.
My whole leg reach, pedal stroke and therefore my balance were majorly affected. Out of habit, I'd go to pull up or push back on the pedal, and my sandal would fall off. I made it till the very end, where we had to walk the bikes in through a narrow space in a gate. Re-mounting, I lost my balance, compensated with a body lean, and hugged the chain ring with my calf.
But it was a really nice ride, strolling easy in the deepening dusk and cooling air and wafting breeze. I hope we go again. But I'll wear my bike shoes.