I'm done with Contis

Well, I realize it may be just a coincidence but the experience with Continental I've had so far makes me look for sth different.
I've been busy lately so this spring wheel change came late. I store my wheels properly (they hang in covers and are rotated) so I was shocked to see what had happened to one of the summer tyres:

My first thought was that it must have been a rodden or something but the whole tyre is kinda flaked off evenly all around and the rubber looks... rotten on the biting edges(?):



Have you ever seen anything like that? Any idea what might have happened to it? I couldn't have overlooked such damage while taking the wheel off in autumn. Ok, I know these tyres are old but the rest of them look really fine and they still have 5-6mm of tread. Can anything like that happen just because the tyre is 6-7 years old?
Anyway, I have just ordered a brand new set of Michelin Pilot Sport 5.