There is a evry low chance that a large amount of pigs spawn on certain biomes.

Known Biomes

• Plains

Noticed this too, but didn't realize it was related to this mod. I sometimes see a lot of vanilla pigs in the Snowy Tundra biome, but I have changed a lot of the default biomes for mob spawns from this mod. I do have the Snowy Tundra as a spawn biome for Pale Pigs, if that is helpful.