People used to think it was to do with the mum being too rough, and some breeders still say that this is the case -usually ones who are trying to deny any fault with their lines! Since modern day science has proven this not to be the case and as many people who have caesareans or cut the cord themselves still have pups with hernias, it is obviously more than rough handling.

It can be resolved easily by a vet - most prefer to wait until spaying, and will perform the ops at the same time.