Tread separation is the process through which a tyre (for various possible reasons) separates into its component parts. Running for a considerable time in an underinflated or overloaded condition is the single largest cause of tyre separation.
It is most commonly illustrated by the tread sections of damaged truck tyres seen discarded on the sides of UK motorways. This is usually due the tyre suffering a slow deflation, and losing pressure, until the volume of air inside the tyre is insufficient to carry the loads and stresses placed upon it. As the tyre pressure drops, the build up of heat within the casing leads to delamination and the component parts separate, resulting in a dangerous (and possible fatal) tyre blow-out.