Shirts come untucked for a variety of reasons. It most commonly occurs when someone reaches up, extends their arms upward, or sits down. Shirts generally stay tucked when your arms are resting by your side, so when extended, it pulls the material upward and out of your pants. Since the waistline of your pants fits closely and is more taught when wearing a belt, your tucked-in shirt can’t slide back down into your pants without something pulling it back down.