What is kitten season?
Why does kitten season occur? Because too many kittens are born when cats who are not spayed and neutered mate.
The easiest way to help reduce the overwhelming numbers of unwanted cats is to spay and neuter your own cat and encourage others to do the same. Unaltered cats are driven by their hormones and tend to sneak outdoors primarily in search of a mate. Mating just once can start a domino effect that can result in dozens, even hundreds or thousands of unwanted animals.
Above information is taken from the Humane Society.