This was an act of mourning. It was something you did when you were in the throes of sorrow. Now it is certainly possible that the sorrow was for the fact that Benjamin was found with the cup and would become Pharaoh's slave. But again, I think there is more than this. These brothers knew God was exposing their guilt.
Repentance requires sorrow for sin.
It involves, sorrow that we have offended a Holy God. I think there is a genuine humility in the brothers. They take the posture of people who have no hope except to plea for mercy. They don't come presenting their credentials . . . The truly repentant person does not come trumpeting their goodness.