[EricH]

On the contrary, it is not a descore. <G25.1> says:

<G25.1> De-scoring GAME PIECES – Once a GAME PIECE has been SCORED, it may not be intentionally de-scored (e.g. removed from the TRAILER). De-scoring a GAME PIECE will cause a PENALTY to be assigned. At the end of the match, any intentionally de-scored GAME PIECES will be considered SCORED as originally placed. GAME PIECES that are knocked free from tenuous placements as a result of normal game interactions (e.g. a GAME PIECE on top of a pile of MOON ROCKS that completely fill a TRAILER falls off when the TRAILER is bumped) will not be penalized.

I would call the top of the target a tenuous placement; a swift hit to the trailer to move it would easily cause the moon rock to fall, whether into the trailer or not would depend on physics. No penalty.