Seat post bolt sheared off - twice
The photos look like you had the saddle clamped at the very rear end of the rails. This, especially if you sit at the tip of the saddle like time trialists often do, puts unusually high torque on the seat rails and clamp. In your case, the rails were stronger than seat clamp bolt.
Since there are four options for mounting the seat clamp, try to choose the seat clamp position so that it is close to center of the saddle rails to avoid unnecessary stress on the seat clamp.