after some experimentation, I have a theory for what is happening:
In Kerbin's gravity, the landing legs supporting the pieces are compressed somewhat. When the docking port is undocked, each leg imparts a force on the craft proportional to how compressed it was (so it would seem)
I downloaded HyperEdit and tested changed the gravity to Minmus levels. The landing legs were barely compressed, if at all, due to the decreased gravity. I did not notice any parts jumping around or exploding or undocking, so that's good enough for me as Minmus is where these pieces are headed anyway!
Thanks all for the input!