The season could be it because the under piggy was chasing the dominant piggy with butt sniffing which is not like her behaviour but we did see it a bit a while ago. For example while they were out on grass under piggy was in a hidey teeth chattering and squeaking loudly while the dominant piggy was doing that outside. Whenever under piggy tried to leave the hidey, dominant piggy ran to that end. When she did get out they began circling slowly with more chattering then did hair pulling. At the moment they're separated in the upstairs and downstairs of hutch with the stairs closed. Normally only the under piggy can go downstairs, the dominant piggy doesn't know how to go down the stairs, she can only go up if we put her down. So that means under piggy can spend quite a bit of time away from the other one.