Well the basic thing going on is "acid + base". Baking powder contains both (baking powder is baking soda + cream of tartar - https://en.wikipedia.org/wiki/Potassium_bitartrate), and starts reacting when water is added (try it).

Baking soda is a base, and reacts with buttermilk which is acidic.

It's not clear to me why some recipes contain both acid+baking soda and baking powder. Maybe they're worried that the first combination isn't sufficient?

My breakfast this morning used yoghurt + baking soda ;) (бухти)