I hope someone can help with this

After taken a pregnant stray in i ended up keeping two of her 5
kittens as well. The kittens are now 7 months old and about two months
ago the mother started being very hostile towards them. No problems
before, she had been the perfect mother to them up until that point.
I put it down to the fact that as they were both toms and were not yet
neutered and she felt threatened by them.  They have now been neutered
and mother is worse than ever towards them both. They only have to
walk in the same room and she is going for them big time. I feel so
sorry for them because she was such an attentive mum and they seem so
confused by it all.

Any ideas why she does this please?