Full disclosure: I'm using RTG and Just Another Spawner, which obviously changes the spawning logic.
However, unlike other mobs, foxes seem to just accumulate nonstop in their home biomes.
JAS is smart enough to replicate default spawning behavior so that you can modify it to your liking, so there's probably something going on with fox despawning that is not working properly. Maybe it's something to do with despawn radius?
Sights like this are very common, and only with foxes.