Magnetic tape degrades by known chemical processes.
The binder systems used in today's audio and videotapes are generally based on polyester polyurethanes. These polymers degrade by a process known as hydrolysis - where the polyester linkage is broken by a reaction with water. One of the by-products of this degradation is organic acids. These organic acids accelerate the rate of hydrolytic decomposition. Furthermore, the acids can attack and degrade the magnetic particles.