Why does my ice cream get crispy in my float?
I think I know what you mean, but is the ice cream "crispy" only on the surface? Unless your ice cream becomes crispy in its interior as well, (which I could not explain), I think it's simply as you speculate, the water in the soda is freezing into a crust of ice around the surface of the ice cream ball. This would explain why it is more noticeable when the soda is cold.