I've noticed that the pigs dig up pavement under houses and walls. Before this wall was perfectly flat, and it seems like the pig dug under it causing the tiles to sink. They seem to dig randomly when pissed, and also right after give them unlubed medicine.
This could be a good way to grief someone: Bring a bunch of pissed off boars to someone's nice brick wall and then shove unlubed supplements up their bums.