Not a manufacturing fault

Bought shoes from Spitz last year July and only wore the shoes 4 times. The last time I wore the shoes in April 2015 the left heel broke. I took the shoes back to the store as I couldn't understand how the shoe could break where it did. The response I received was that it was not a manufacturing fault and I stepped badly on the shoe and that's why it broke. how badly am I supposed to be stepping for it to break in the manner that it did?<br> I would like to know how they got to that conclusion. Had I been wearing the shoes frequently I would understand.