I believe my dog used to do it when he felt sick. I am deducing this because I spent a lot of time outdoors with him, and he never ate grass as a regular thing. Didn’t interest him. But on the rare occasion he did, he would soon puke and wouldn’t be very active. I can’t really know for sure what he was feeling but I am assuming he felt sick in his stomach and some deep dog instinct told him to eat grass to help him spew up?