My Aby does that too. But for him, he just does it out of spite. Like if we are cleaning his ears or clipping his claws, that night he'll poop right in front of one of the boxes. Also, if we don't feed him when he wants to get fed, he'll do it again. It doesn't matter if the boxes are clean or dirty, he's just spiteful. We've pretty much given up on trying to do anything about it, we just clean up the poop and go about our merry way.