Something strange is going on with body mechanics. Intertia is sometimes not conserved in collisions, especially if the colliding bodies are of similar size, i.e a moon can escape a system at more than twice the combined inertia of the colliding bodies. Furthermore there is a bug with the 'Lock Position' function whereby the system attempts to conserve inertia by giving orbiting bodies inertia instead of making the contribution to the locked parent null. I made a compact solar system using a locked Saturn, and following a collision, all the orbiting bodies rocketed off in a concerted direction.