I think what happens is that the factory UV coating reduces over time, and this process is exacerbated when you sand them back to clear. Sanding makes them clear but likely takes off all the UV coating, which is probably why the headlights return to their previous haze/craze more quickly than the first time.

This is why protectors are so critical. I'm eager to get my headlights fixed and covered with Stongard, hopefully that will take care of the issue permanently.