Of course there is a logical explanation. Every time Isaac uses the potty chair, he needs to be naked. After using the potty chair, he does a victory lap. And any time Isaac is running around naked, Drew must follow suit (birthday suit, that is). It all makes perfect sense. And this reality is making potty training Isaac even more eventful than I anticipated.
Wait a minute- when walking in on this scene makes perfect sense to you...I don't think I'll finish that thought.