Thanks! The rear camera is a cheapy Midland XTC-100 that I bought from REI for $64. It only has standard-definition resolution at 640x480 30 frames per second, whereas the other two cameras are still SD resolution but record at 848x480 and 60fps. It could be the lower frame rate causing the weird effect, but I'm not positive.

I've seen the same effect on other people's videos where the wheel is visible.