I think we need to get one thing out of the way first and that is whether mother cats sometimes kills their kittens to eat them or whether they sometimes eat their already deceased kittens. The former, infanticide, has only been documented a few times with respect to domestic cats and therefore I will have to proceed on the basis that the question refers to mothers eating their own kittens when they have died for whatever reason.
The answer to the question must be exclusively to do with survival. It seems that almost everything that wild animals do (and the domestic cat is essentially wild at heart) is driven by the desire to survive.
“They will kill them when they sense a defect and know they will not survive; they may also do so when disturbed by wildlife or humans,!or if they have no milk.”Pam Jackson
The queen (the mother cat) expends a lot of energy and effort during pregnancy and afterwards in bringing kittens into the world and in raising them. If a kitten dies at birth or shortly thereafter, on the basis of a strict almost mathematical equation, it makes sense for the mother to eat the deceased because it’s a form of energy. We know that mothers eat the afterbirth which is a similar process and the motivation behind that is also similar.
Foetuses are sometimes reabsorbed if they are unviable and die before birth. This is also somewhat similar in terms of the underlying reasons for the process. The experience below indicates a cat who is mentally disturbed or she is believes that the conditions are too harsh for her kittens:
“I have a feral – the only one I haven’t been able to trap yet. She is very well-fed and has plenty of comfy houses and places to hide. She had had 3 litters, and has killed them all. She killed the first litter when they were 3 days old, the second litter at two weeks, and the third litter at birth. She is just insane. I can’t wait until I get her in a trap to be spayed.”Tammy Campbell on Facebook
It’s also been suggested that if a mother is stressed and anxious and eats the afterbirth to recoup some of the energy expended in giving birth then she may keep eating. This suggests that the mother will kill her offspring accidentally.
“We had a foster mother that ate one of her kittens. She had moved the litter to a dangerous location and we moved them to what we believed was a safer place. In moving them back, she killed one. I have no idea if it was accidental or she was just confused. After that we confined mothers before they gave birth. If they came to us after they’d delivered, we confined them but kept a very very close eye on them in the first few days.”Laurie Cavanaugh Bobskill on Facebook.
This is perhaps more likely to happen for a new mother finding the experience stressful and confusing.
There may be another reason for a mother eating a dead kitten. It may prevent predators coming around to feed on the kitten. That would endanger the mother and the other kittens. Obviously, in reality, this would not affect the domestic cat but, as mentioned in terms of motivation and inherited instinctive behaviour, domestic cats behave very similar to their wild cousin, the North African Wildcat.
These are my personal views. I have not been able to find any details in research on this subject except a woman responding to the question on Quora.com refers to a mother suffering from stress and anxiety. I thank her for the information.