I've just noiced that mine too have done this recently. My PHEV has only done 32,000 miles from new. Poor quality rubber/plastic - too thin perhaps. They weren't like this last November when I put my winter wheel 'n' tyres on. I don't know if perhaps jet wash usage over the winter may have accelerated the deterioration of the rubber. I work in a garage and do use a big, powerful 'professional' use only jet wash. But I'm very careful with it as I'm aware of the damage that can be caused ie, air-con condensor, tyre, rubber body trim and even paint damage. Doesn't look like it would be very easy to replace the gators though... I'm guessing this isn't an MoT failure?