Kerbals cause ship acceleration in EVA
When a Kerbal is on a hatch or ladder while in space they cause a forward thrust or acceleration to the ship they are attached to. The effect is in the direction the Kerbal is facing. No fuel is consumed.
The effect is the same for all typed of Kerbals and regardless of having RCS backpacks or not. The spurious thrust is small but enough to be noticeable within seconds. This effect changes orbits and spins ships depending on the location that the Kerbal is holding on to. It effectively makes on-orbit repairs difficult or impossible when two ships are in close proximity, since the ship with the engineer immediately spins or is pushed away.
I think there is a little workaround. It looks like this effect goes away if you move a little bit. It stops accelerating the craft when kerbal "sticks" closer to the ladder, opposed to floating a little bit above it when you just come out of the hatch.
The effect is reduced, but not eliminated. Wait for a while and watch the orbital elements. They will still change, just slower.