You said the kittens are now 6-8 months old? Although mama cats usually wean their kittens long before the 6 month stage, up to that point they are usually still teaching them to hunt and partially hunting for them. If your cats were feral and on their own, this is the point that mama decides they know enough to feed themselves and chases the babies away to go make their own lives.
6 months is the time my cat decided her kitten was now grown up enough to go and started a campaign to make him leave that included growling, hisssing, and swatting him so hard you could hear the thunk from across the room. She mostly desisted once he completely stopped trying to nurse and gave up bugging her to play with him, but still reserves the right to box his ears if he does anything she considers wrong. (Like looking at her funny, sitting in the spot she wants,washing her ears,etc.)
Dr. Seuss~DLH (brother's cat)~June 2007-
Misty~DSH (my cat & Mooby's mom)-?-
MooBoots(Mooby) ~ DMH(Mom's cat)~July 21, 2008-
Sunshine~ Golden retriever X white german sheperd (Dad's dog)~October 24, 2008-