FranknStein
Tuned and Synch'ed
I am trully sorry to hear this. It looks like that happened while tumbling on the pavement. What do the "hooks" at the end of the two mounting posts look like, and also the "spikes" on the bottom of the passenger seat bottom that secure the two posts/hooks from moving rearward? Any unusual wear marks/points? Did you by chance spray any silicone or any other type of lube as someone suggested? I think that is not a good Idea... Lube will attract grime and turn into a fine lapping compound. Tight fit is almost always best.
There are some minor cosmetic marks on the hooks but nothing concerning. I never put any lube on the mounting points. I did a bit more investigating, and there is simply no way the panniers weren't properly mounted when I set off this morning. Something is awry here. My best guess is that crack actually wasn't from impact. If that mounting point had already been weakened, it could have cracked while riding. The crack causes the pannier to sag. I can imagine how it could have slid out if it were sagging.
The whole mounting system is made of plastic, and the panniers require a bit of elbow grease to get on and off. Maybe a crack had already formed.