I visited my local pig village during the day to find the last pig man being slaughtered by three werepigs. I have no idea where they came from. I ran for it but couldn't get away from all three. I've seen them transform when fed monster meat and during the full moon, but don't know of any way for three to appear in a pig village during the day, but obviously want to avoid similar confrontations in the future. What caused the Werepigs to appear?
Basically, if the werepigs are busy (probably killing something) at the moment they're supposed to turn back to normal pigmen, they'll stay in their current condition, attacking on sight anything near them, be them spiders, tentacles, Wilson or even their own kin.
Well, sometimes, Werepigs will stay in that form until the next one to three days. They will be found sleeping outdoors (USUALLY!) when they transform into Pigmen again. If they are chasing you and transform again, they will usually fall asleep, then the next day, run to their village. Hope this is verified information!