I can tell you 2 reason off the top of my head.
1. there could be an invalid Spark animation in the hitdef. If the def doesnt regognize the animation it would create a clone on the hit.
2. there could be an incomplete hit overide where if a move is blocked, or hit it is supposed to go to specific hit state. If that state is missing it can create a clone.
There are other reasons it can happen too.