It's not natural to tuck your chin to your chest, but it is an effective way to protect this vulnerable part of your body in a fight or even a fall. Very often while fighting, especially when tired, we stick our chins up.
When you get punched on the chin, it can rattle your brain and the fluids in your ear and cause you to get dizzy and off balanced. Here is training tip that might help you to condition yourself to keep your chin tucked in a fight.
You can do this while working out on the heavy bag or while shadow boxing or even pad work.