Why did Mother Earth send the monsters to kill Zeus?

If you are referring to the Percy Jackson series the second set of books; she was mad at Uranus for sending her first 2 sets of children into the depths of tartarus and then conceiving the titans. Gaea loved the titans, but when the Olympians overthrew the titans that made her angry, so she unleashed her first 2 sets of children on the Olympians, mainly Zeus.