OEM winshields are horrible when it comes to rock chips. If you do a search, you'll see that this happens very often (mostly with OEM). In fact, I had to replace my oem windshield one week after I bought my car new. After I replaced the oem windshield with an aftermarket one, I have not had this problem anymore. The ones that the mobile window guys replace with are actually a lot stronger, for some strange reason.