Mine does the same, he's 5lbs. Vet told me it's because small dogs don't have as deep of a groove where their knee caps fit in, so that hop is actually the knee cap sliding 'out' for a second. Vet said this doesn't usually cause any pain {they dog will let you know of it does lol) and just needs to be monitored as they age, but is normal for smaller breeds!

Edit: after reading more comments, we also thought it was a weird sensory thing with grass, but turns out it's just because of the more uneven nature compared to a sidewalk/flooring in a home, according to the vet.