Samething happened when I snapped the fram on my Lemond (mfg'd by Trek with Bonti components).
The bracket hardware of the saetpost is plastic where most other psots are made of alum or other more durable material. I fixed 'er up but it continued to slip after the hard pounding. I removed the hardware, flipped the bracket type block (above the rails) so that the unstripped section faced formward. Hasn't happened since.
If the post is Bontrager, maybe the same situation. Maybe cause by unproper torque and now stripped. My other post of the other roadie is Thomson. Expensive at $80 but never had a problem like this!