Resupply missions are moving up earth's gravity well. If an engine malfunctions, the craft will lose velocity and altitude due to Earth's gravity. Return missions are falling into Earth's gravity well, so engine malfunction results in continued acceleration. Also, retropropulsion is fundamentally unstable (the force balance is the same as balancing a ruler vertically on your finger), which means that if a system loses attitude control, the craft will much more likely enter a tumbling condition, which if not arrested, will dramatically widen the cone of possible collision.