Wiper problems of the 2007 Mini Cooper S

Several weeks ago I found a crack in windshield of 2007 Mini Cooper S on passenger side below wiper. I had the windshield replaced and car was returned to me on sept. 10th. On sept. 16 I found another crack in the new glass in approximately same spot! after looking on internet I found that stress cracking of the windshield is a common problem with this car. Anyway, the dealer claims it is an impact crack and only a coincidence that it occurred in same spot! after arguing they agreed to replace the windshield at their cost--as a courtesy. I was not aware of anything hitting the windshield and even if a small pebble hit--the windshield should not crack so easily.