It's pretty common on cars that aren't used much. Back in the "good ole days" when tires only lasted 10K to 20K miles you didn't see dry rot problems that often, the tires would wear first. These days if you have tires that last 40K-50K miles and put them on a car only used a couple thousand miles a year or less those tires will rot away well before the tread wears out.

wow. I've never seen that here in Canada. When you say 'rot away' do they actually rot right through, or they just look bad?

I see it all the time since I have some older vehicles that get little use to where it would take decades to wear down tires with 40,000 mile life expectancy. So the tires suffer from dry rot way before they wear out. I've discarded tires that still look nearly new as far as tread wear.

I guess they'd crack all the way through if you let it go long enough (I've seen that happen on sidewalls), but before that you'll see cracking in the tread area. It's not just cosmetic, it's a serious safety problem. Tires can also rot internally where you don't see it.

A big part of the problem is exposure to strong sunlight (UV rays), especially for cars that are stored outside, and you get a bit less of that up north.