Pokemon Sun and Moon?

[Chaoxys]

I don't understand how being on this tropical island causes a fire-type pokemon to become ice type?

If anything you'd expect their fire abilities to just get stronger, since it's essentially like Sunny Day is active ALL THE TIME.

Their entries on the website seem to imply the Vulpix and Sandshrew line had to escape to the icy mountains and adapt there.

I really like that they are going out of their way to explain why pokemon have new typings. I mean, the Alola forms are almost like brand new pokemon in everything but name. That said, I hope the new forms get their own dex entries.