Iphone 6 screen cracked while charging?

I originally bought a new iphone 6 from a store with warranty and all about a month ago, then a week ago, I went to a random store and exchanged my iphone which was gold and bought the space gray one because i liked it better, and paid 75$ for changing it, yesterday night, the phone suddenly cracked while I was charging it, it didnt drop, no one even touched it, it cracked in front of me by itself. Any ideas on how this might have happened Or what I can do now? Any help would be much appreciated. Thanks