Tar peels when it loses adhesion to the surface beneath it. The usual causes are moisture, dirt, old coatings, smooth surfaces, heavy application, or movement under the tar layer.
Peeling is an adhesion problem. A tar coating can only protect the surface it actually grips. If it is applied over dust, wet wood, loose paint, algae, frost, oil, or a polished sealed surface, it may form a skin that looks complete but is weak underneath.
Moisture is a common reason for peeling. Water can enter from cracks, joints, end grain, or the back of a board. Once it sits under the coating, it can break the bond. Sun then heats the dark tar, pressure changes, and the edge begins to lift.
Heavy coats can also peel because the outer layer dries or skins before the lower layer has settled. The coating then behaves like a separate sheet rather than part of the surface. Movement in timber or roofing can pull that sheet apart.
Peeling tar needs more than another coat over the top. The weak material has to be understood. If the reason is damp, dirt, or poor bonding, covering it again repeats the same failure.