Some pieces are B.A.D. (Barely Adequate Defended).
When a piece has zero defenders and zero attackers, we call that piece unprotected. When you add an attacker, that piece is hanging. LPDO, remember?
When a piece has one defender and one attacker, it is effectively unprotected as well. When you add an attacker, that piece is hanging.
When a piece has three defenders and three attackers, that piece is unprotected as well. When you add an attacker, that piece is hanging.
Which piece of the the three examples offers the greatest chances for a tactical combination? The piece with the most defenders, of course. Since not only the piece itself is vulnerable, but the defenders, which are immobilized due to their defensive function, are vulnerable as well. If you manage to eliminate just one defender with tempo, the defended piece is in trouble. So develop an eye for the most defended unprotected piece.
That is the rationale behind overprotection. The defenders are no longer immobile, and hence no longer vulnerable for tactics. Neither is the defended piece.