Normally that shoulden't happen but I guess your dog could be under extreme stress at the time. Is this the dogs first time birth...?

During the birth of my dog toby (picture to the left) the family setup a basket with warm blankets and family members was on shift checking. During the birth we didn't touch the pups immediately but let mommy lick first... only later do we touch the pups and help mom rub their bodies and limbs but with the blankets. The rubbing helps stimulate the pups to get moving and we can place them on tits to start suckling.

Since it was Jill's (mommy dog) first birth it was supervised by people to avoid problems.

Anyway another reason why a mom would eat her pups is the pups are all still birth...