It happens every time a start FSX, could take 5 mins or more to complete
Your log shows it took 137 seconds. However, that's not the issues, the cache regeneration should happen ONLY if you add a new scenery, or make any changes to the Scenery Area (like re-arranging areas, removing areas, etc.).
If you didn't do it manually, it might still happen if you have some kind of external add-on product that makes changes to it automatically, like a scenery.cfg manager, an atc program that creates AFCADs on the fly, etc.
Another reason it might happen, is if there were errors in the previous run of the regeneration. Your log shows several Warning messages about several AFCADs described in multiple BGL files without proper DeleteAirport record, but this isn't usually a problem, it's just an indication that might help you solve possible conflicts.
