2 thoughts:
First is that over time, rubber breaks down and loses its properties. That's because the molecular bonds are broken by 1) being attacked by oxygen and 2) due to stress. The result is cracks.
The photo - while not a photo of the tire in question - is typical for the early stages of cracking. Where that particular tire is in the sequence is the point where there are stress marks just beginning to turn into cracks and nothing to worry about. That lower sidewall area is a common place for those types of stress marks to appear first and will not cause a problem until the cracks become MUCH more severe.
And, No!, it is not because the tire is *drying*. It is because of the combination of the stress, the amount of flexing that takes place, and the material properties of the rubber in that area. The way to prevent it is to properly inflate the tires, and keep them from sources of heat and ozone - which is hard to do when they are mounted on a car. - AND - once the rubber starts to crack, there is no going back. Many of the tire dressing include antioxidants to provide and alternative to the oxygen attacking the rubber, but nothing can be done to repair the rubber breakage once is has occurred.
