Phone was in left front pocket of a loose fitting jeans. This is where I always keep my phone, have gone without problem for years with iPhone, Xperia S, Nokia Lumia 900... never had a problem. Back of phone is always facing outside.
I tend to have the phone there and keep my hands in my pocket when i walk - only thing I can see is that my hand put a little pressure on the back glass and caused the cracking as I am positive I did not it anything and never keep anything else but the phone in that pocket. This still seems to me like a design problem because there is no "unnatural" pressure or wrong use here for a cellphone. Back glass just cracked all over, there is no clear breaking point that would indicate a shock, just cracks on the entire surface of the back glass.