The reason why female octopuses stop eating after giving birth is due to a complex biological process that they go through.
The complex biological process that female octopuses go through that results in the octopus stopping eating and wasting away and dying is mainly driven by the optic gland which is a structure that is similar to the pituitary gland in mammals.
The optic gland in the female octopus undergoes a dramatic shift in it's cholesterol metabolism, which leads to the production of hormones which trigger a cascade of self destructive behaviors.
The behaviors include ceasing to eat, caring for the eggs until the eggs hatch and ultimately death of the female octopus.
The optic gland in the female octopus is located between the female octopus's eyes and is responsible for regulating of various different bodily functions, which include maternal behavior and reproduction.
After the female octopus mates and lays eggs, the optic gland releases a surge of hormones.
The hormones, most particularly the hormones related to metabolism of cholesterol, drive the mother octopus to focus solely on caring for her eggs.
And this focus on the egg care often results in the mother octopus neglecting her own needs, specifically eating.
And the hormones also seem to trigger a decline in the overall health of the female octopus and a tendency towards self harm.
Octopuses are also semelparous, which means that the octopus only reproduces only once in their lifetime.
The self destructive behavior of octopuses following reproduction is an extreme example of the semelparity, ensuring the survival of the female octopuses offspring at the cost of their own lives.