If the lower lip rests behind the upper front teeth, then it will be sucked into the mouth to make contact with the tongue on swallowing. Everybody swallows about 2000 times a day.
The more the lower lip is sucked in behind the upper front teeth, the more the top front teeth will be in front of the bottom front teeth. This is the cause of “buck teeth”
The lower lip always sits behind the top front teeth, producing “buck teeth”