Why do mother cats become hostile to their kittens once the kittens reach adolescence/adulthood?

Not all of them do but, in general, the reason for it is due to their job as protector and teacher is now over, and it's time for the kitten to go out on it's own.  You see the same behavior in big cats in the wild.

This link might help explain it
https://pets.thenest.com/kittens-along-mothers-adults-11308.html