There is a hill on this street the pigs cannot quite climb over, causing them to never ever reach the trees to get fed naturally. Only pigs on the left side of the hill can regularly be nibbled without being hand-fed constantly because of this.
Suggested fixes are to either fix the pathing so the pigs can get up and over that hill... or add or move a patch to the other side of the street where the other pigs are stuck.
EDIT: I'm starting to think the trouble has to do with the mini-ladder thing going down into the little pit there just where the hill starts going downward. Chickens and Pigs will not cross that particular spot.