Cracked flywheel

I have a GMC Sierra 1500. Just ten months ago I had to replace the transmission. A little while ago I noticed a sort of clicking whenever I accelerated and then it would stop after the shift change, well I really didn't think too much of it. Now the clicking is clanking and it's much louder. I just took it to the dealership that replaced the transmission to see if they could tell me what it was. The service man diagnosed it a crack flywheel. What caused this flywheel to just crack.
The truck had just gotten to 100, 00 miles when the tranny got replaced. I don't believe the job was done right.