There are other explanations for such happenings. For example, there could be several passengers inbound on an int'l flight where the flight you're on is a common domestic feeder route, and the int'l flight is late. All of the J/F passengers on that inbound flight will get free domestic F seating on the feeder flight - perhaps there were several that were ticketed on an earlier feeder flight and have now been moved to yours, thus taking up many of the F seats. Delays in long-haul int'l flights can be known precisely many hours in advance, and in some cases even more than a day in advance if it's a crew issue.