A while ago I noticed cracking around the edges of the rear tyres on my 212 estate. The tyres are the original Bridgestone potenza re050a and are 4 years old covering 20k miles. The cracks run around the circumference on the shoulder where the tread meets the sidewall. Today I took the car in to have a slow puncture fixed and two tyre places refused to fix the puncture due to these cracks. I have had a pair of new tyres fitted but am wondering what caused the cracks. The front tyres show no signs of cracking so why the rears? Is this common on these tyres/cars?