At some point just about all mother cats, whether spayed or not, will wean their kittens. I'm no expert, but from what I've read and observed, it's normal behavior for a mother cat to "enforce" her offspring's independence sooner or later. Instinct probably leads her to get ready for her next litter (she doesn't "know" there won't be one once she's been spayed), or perhaps it's nature's way of preventing inbreeding (mother-son incest).