Today I was out washing the car and noticed my stock Dunlop tires on my GTS wheels have cracks all over the tread. I've done about 15000km's on them and had used my winter tires for the rest of the time, but notice this on all of them.

I've always maintained proper tire pressure, even rotated them, stored them properly over the winter but I don't know why they are like this. Has anyone noticed this?