I have crashed my bicycle a few (hundred) times through the years. The great majority of those crashes happened on a mountain Bike, and I consider crashing to just be an unfortunate part of riding off-road.
Of all my crashes on the road, I can tell you a reason for all of them except for one. Mostly, road crashes occur when I hit something which upsets my equilibrium. You know, a patch of gravel, some wet leaves in the apex of a curve, or some ice gets in the way...
But, one day, I had a crash I still can't account for.
I was riding my Raleigh SuperCourse fixed-gear to work, at Campus Cycles, from my apartment at Parker Road and Orchard. I had already passed the house I was to buy, three years later, and I was at the intersection of Iliff Avenue and Colorado Boulevard. The light was green as I approached, then turned yellow as I entered the intersection. I stood up to accelerate through the intersection and...
And... the next thing I new I was over the bars and crashing into the pavement in the middle of the north-bound lanes of Colorado Boulevard. I saw my pirate-skull tail light bouncing across the road in front of me, and a garbage truck approaching from the south at what looked to be a pretty high rate of speed, just as I was registering the pain in my knee that I had landed on.
I disentangled myself from the bike, picked it up, and ran/limped to the far side of the intersection. Once I was on the west side of Colorado Blvd, I set the bike down on the sidewalk and went back out into the street to recover my tail light.
I checked the bike out. Nothing seemed out of the ordinary. I looked at the street surface, and I could see no hole or object which could have stopped my front wheel and caused me to flip over. I could not come up with a reason for my sudden launch subsequent splashdown in the street.
I put my tail light in my pocket and rode on, trying to come up with a reason for the crash. But, I never could. It's still a mystery to me.