WDW Mickey stepping stone

[wisbucky]

We purchased several of the Mickey Stepping Stones during Flower and Garden Festival. The stones were placed out in the garden. The next morning one stone was cracked from the center outwards. No one stepped on it no one set anythign on it. I am wondering if anyone else has had this problem or if anyone is aware of why these crack like this. I am now very concerned that the rest of them will crack the same way given time.