There is nothing in the script, but tensions in the relationships between all of them get laid bare thoughout the second half of the play. Their vulnerability at that moment is part of the progression traced in the script.
Given the wooded environment, the tearing of clothes is a convenient and obvious way of making that vulnerability physical.
Other productions have had them getting wet or mud caked.
All of this comes though from the same impulse--to make the audience feel the how "exposed" they are by their experiences in the woods.