Exact same thing happened to me two weeks ago on a 150 mile ride from Alabama to Georgia on the Chief Ladiga/Silver Comet Trail. After 30 miles we stopped for lunch in Cedartown GA and when we mounted up to continue the trip my rear wheel dropped out of the drop outs (very appropiate name) and locked up putting me on the pavement with right foot still clipped in. Result was badly skinned right knee and sprained left wrist, of course several spectators watched the spectacle from the sidewalk and street. Made the next 40 miles kinda uncomfortable. Problem was totally my fault, I had installed a new rear rack prior to the trip and apparently did not tighten the QR enough. Lesson learned, I always prided myself on being detail oriented so this left me somewhat upset. I always preached to my daughters to check everything before riding so I have had a lot of grief from this incident.