I agree with this, also suspect the pig may have something to do with things. Even if the pig is gone, his smells remain.
May sound stupid, but I have a gelding that is TERRIFIED of pigs. I went riding at a friend’s house, her adult pig got loose. I had to pull the horse trailer down the road a significant distance in order to get my horse calm enough to load. The pig is long dead, but the smells/memory remains. Every time the horse has been back to that property, he watched for that dang pig.
An old outfitter told me pigs smell like bears, and his horses hated the smell. He also said he used to butcher his hogs and try to desensitize the horses to the carcasses (smells) to get them ready for hunting so they were able to use the horses to pack out game, including bears. not sure how true the pig/bear correlation really is, but after watching my horse absolutely lose it when he was approached by a 600# hog, it sort of makes sense.
Or maybe my horse is just 1/2 chicken…
Although, I have seriously considered finding a pig to borrow (or even raising one) just to desensitize my horses.