I have the same thing on the Conti GT4000 and I don't like it. I find it worrying. I have other tires where the rubber looks like new until they are worn out. I suspect it has to do with the type of rubber compound because I had a set of tire on my auto with the same cracks but other tires on the same vehicle looked fine 'till the end with 50,000 miles on them. I have one Conti GT4000 now, a folder with plenty of rubber left that I only use as a spare.