It happens.

Also keep in mind that though it's only 1-1/2 years old to the user, it might be much older in real time, because we never know how long it sat on a hook someplace before he bought it.

Belted tires are prone to this kind of stress cracking of the rubber, especially if there's a big transition in thickness close to the edge of the belt. Weather and UV aging is also a factor as it makes rubber lose suppleness.

Add high loads and/or low pressure which cause maximum flex at the contact patch, and voila. But as I said, it's 95% cosmetic, and odds are that the tire will last long enough for something else to kill it off first.