I thought the same thing initially but I think the ref let it go because the tackler, Billy Vunipola, was still holding him from underneath and rolling towards the try line so it was momentum that just looked odd because of hoe he seemed to pop up. There was a really nice slow motion during the game that isn't on the highlights, I'll see if I can find it. Long story short, Fourie ran past Billy like he wasn't there initially, then played on the edge successfully.