Not that it explains much, but there's a previous sequence/image where you see Takato start baking and he turns into an impmon (the snickering/lighter purple one) and taints the food with jizz before losing his penis and watching the changes start with Rika while she eats.

Nothing in the first one says why he changes, just... adds a little more context

link to picture here: https://www.furaffinity.net/view/22266271/

Base +8, Yokou +6