Windshield problems of the 2008 Toyota Yaris

I bought my 2008 Yaris brand new in June of 2007. This year, sometime in July or August, my front windshield spontaneously cracked. It was sitting in my driveway, safe from harm, when this happened. About a week later, the back windshield shattered. The service people at Toyota of seattle confirmed that the crack was not due to impact by object or force because of the pattern of the glass break. Toyota also took responsibility for the back windshield because of the odd shatter formation of the glass. There was no point of impact in the glass, and the glass shards fell straight down, instead of forward or backward which is consistent with blunt force breakage. Toyota took responsibility for all charges and for the repair of my vehicle's windshield. I am satisfied with their customer service in this matter, however, I am constantly worried that my windows could break again at any time, or that it might happen to someone else in their Yaris. Hopefully this was just a fluke as I am highly satisfied with everything else about the vehicle. I just worry about my safety as well as others' safety if this were to happen in other models.