Happens to me all the time, on my ten-year-old PC that was never optimized for gaming to start with.
It's just failed to render the ground properly. The rocks are sitting where the ground actually is. I'm sure if you drive over to where the rocks are floating and do an external view of your SRV, it will be floating too.
Immersion-breaking floating rocks and bases and a "ground level" that's several hundred metres above or below the actual ground is one reason why I don't land on planets very often. The other is the fact that it usually takes a minute or two to drop out of orbital cruise.