I am not a pig expert, but in general, testosterone may continue in their systems for quite a while (can be several months, even up to a year if they were fully developed when castrated) if they were not castrated very early. Plus, in dogs certainly and presumably some other animals too, they produce testosterone in the adrenal glands as well as the testes, and in some individuals, at a level that still gives male behaviours even once the testes are removed. (Again, more so if castrated later, especially if fully developed.)