Some cracks are more likely to happen than others, in extension, perpendicular to the force applied is where it will break first ( boof is the breaking sound apparently) in engineering it is most often extension. Geologists work in compression. What happens when you're pushing the original cracks together makes cracks in the vertical orientation will appear as the cracks in the horizontal orientation close.

How do we know? Milk in glass bottles- glass in refillable bottles but they would break easily and someone wondered why? so they discovered there were microcracks and would make the bottles break easier- not all glass is created equal.