1. THE BRAIN JUST CANNOT MULTITASK AT CERTAIN THINGS
The human brain is a mega computer and has the ability to do quite a lot of things, but sometimes, there are things it just cannot do smoothly — moving both the tongue and the eyeballs is one of such things.
Kissing obviously involves swirling the tongue in the other person’s mouth, and the brain is sort of programmed not to do that and control the movement of the eyeballs at a time. Try it when you kiss again, and see how unusual it would seem.