That one looks really pretty. I don’t remember red velvet being sour but it must be with sour cream or buttermilk, and that white vinegar. I bet i never had a real one, just a mix. Which always rise wonderfully, it’s a mystery.
No mystery about it, actually. It is basic chemistry. The acid in the buttermilk and vinegar mixes with the base element, baking soda. You get the chemical reaction which makes the cake rise.
It is the same thing that happens when you make a “volcano” for a child’s science project, or when you use baking soda followed by vinegar in an attempt to clear a drain clog.