To reproduce, launch this craft:
https://www.dropbox.com/s/quwffls213m817o/RoverDetachBug.xml?dl=1
and activate the interstage above the rover (or simply hit spacebar). Upon detachment, the rover immediately starts jittering around all over the place until it explodes.
If you first take control of the rover's command chip, then activate the detacher, everything works fine.
Why's it spaz out when not under command?
Moving the rover around for more clearance between colliders, or using docking ports instead of side interstages, has no effect.