Tires are constructed with an anti-ozonant compound in them that leaches to the side wall of the tire to prevent deterioration due to the effects of UV radiation, etc. But the tires need to be in motion and under load on a regular basis for this to occur properly. In the RV industry people are told to replace their tires after a certain period of time, rather than after a certain mileage, because these rigs tend to sit for prolonged periods of time. You'll also see full time RVers covering their tires during prolonged stationary periods, also to ward off UV radiation.

If tires do sit for a long period of time some of this cracking can occur, and the use of a tire dressing will often make the appearance more noticeable. Read that correctly - some tire dressings will make the cracks more NOTICEABLE, but dressings won't actually make them worse. Different tire companies use different chemicals, or combinatiion of chemicals, in this anti-ozonant process and this can cause some tires to brown more than others will. Once again, a tire dressing may or may not emphasize this browing, but it won't cause it.

A few years ago I had 4 cars at home, with each running a different brand of tire. All 4 vehicles were treated the same, and all tires received the same cleaning and dressing on a regular basis. Only one car, the one with Michelin tires, exhibited any browning at all, and it was fairly heavy. These were pretty new tires as well. This isn't really good, bad or otherwise, it's just the nature of the beast.

So, if the cracks aren't very deep or widespread, I wouldn't be overly concerned. But if they're starting to look deep, even they aren't huge in number, I'd talk to a tire shop I trust and probably replace them before doing any really spirited driving on them.