I said the exact same thing to them.."Sink doesn't not crack on their own." Their reply was,"Seriously we know a sink can't crack itself but I have no idea what the heck we could have even done to crack it. I don't know a dish in the world that could do that unless you threw it like a baseball...which we don't do."
An extreme temperature change could crack a porcelain sink. If they put a very hot pan or extremely hot water into a cold sink.
Dropping a heavy pot or pan into the sink can cause it too. It doesn't have to be thrown either. Just the act of dropping it because maybe it slipped out of your hand, you "tossed" it in because you were in a hurry or the food was burning and and you wanted to douse the flames/smoke quickly (this would also be an extreme temperature change).
A glass bottle of cologne or aftershave accidentally dropped can cause a crack. It slips out of the hand or drops off the shelf or medicine cabinet if there is one. If they use a hairdryer and it falls into the sink by accident, the force can potentially cause a crack. A glass dropped in the sink can as well. Running very hot water into the cold sink (such as for shaving in in the morning) can cause issues. This is not to say that a crack will automatically happen if you do one of these things. However, these are events than can potentially cause a crack.
Our sinks in our home are 30 years old and no cracks. Cracks just don't happen out of the blue. Something caused it.
Now, depending on the crack, you can repair it with a kit sold in most home improvement stores. Mostly this will work on superficial or hairline cracks.