It's more likely that your granite fabrication was substandard.

I agree with LWO, that it is likely to be an installation problem, Granite can crack along fissures that aren't always visible to the eye. Thermal shock can do it, as can physical shock (heavy object thud), or shifting/settling/earthquaking of the house. Usually, a top notch installation can prevent this, but it does happen.

The good news is that if you haven't shifted the pieces a lot and abraded the edges (and possibly even if you have) a good stoneworker can fix it so that you can't hardly tell that it cracked.