I was fighting a Spider-tron 6000 and a couple Mk. I Sword Robots, and... this happened. I'm not entirely sure how or why, but I think it had something to do with being knocked over (by a Spider-tron bomb).
Interesting find! I've run across this sinking effect a couple of times during testing. If you boost with the jetpack right before the elevator reaches the top of the arena, you can sometimes get this to happen as well. So far there is a bug report filed vaguely along these lines.