That happened to me twice. After the second time I examined the clamp. The two halves of the clamp have these grooves that fit together so the seat doesn't tilt when the bolt is tightened. Mine were worn. I theorized that the seat slipped, backward, I suppose, and thus pushed the top half of the bracket forward, and that force would have been perpendicular to the bolt, and snapped it off. Just a theory. I bought a new clamp (I think I had to buy the whole seat post). Centering the seat in the bracket, in the front-to-back direction, would reduce the force that wants to tilt the seat back.