Although I have heard that tire dressing could be bad for the longevity of the tire, I don't think that it should start to crack the sidewalls.

I'm guessing it's the brand of tire. I had a set of Nitto NT 450's a while back and after a year and a half, the sidewalls were cracking as well. And this happen to be on the side where I can't see the sidewall. Luckily I spotted it when I was rotating my tires.