I've been wondering about that. It is definitely happy when one of my piggies does it. She really stretches out and relaxes. I have another piggie that I'm not so sure about.
One piggie (the happy one) has a higher pitched purr. The one that I'm not sure about is lower pitched, similar to the video. She is very young and new to me, so I can't tell yet what it means. She nips and squirms a LOT when she is unhappy, so I'm leaning towards "happy" for her purring. It could mean annoyed though.
All my pigs let out a purr when I put them back in their cage. It is either "get lost human" or "happy to see my home and piggie friends". Hopefully it's the latter
