This is normal for Michelin, lately...they crack and rot quickly. Someone I work with is still fighting with them after having to throw away $1000+ worth of tires on his A6. They were about three years old, and rotted so badly the car failed inspection. Michelin refuses to lift a finger, instead accusing him (and is emphatically WAS an accusation) of using "unapproved" tire dressing. (He did not.) In fact, his next step is probably small claims court.
His Audi now rides on Yokohamas.