Unexpected aggresive behaviour

[garnet]

Hi All, we have adopted an older 2 year old border collie lab cross about 2 months ago. We were dogsitting a little dog for my uncle. We had our uncle bring over the dog prior to him leaving and they got along great. We had her for 3 weeks and never an issue. Today the uncle was coming over to pick up the little dog and my wife was combing our lab collie dog, and she violently attacked the little dog. So much so that it is currently with the vet. This has upset us greatly and we are now afraid for our children as she has never given any behaviour to suggest that she would be so violent but when this comes out of the blue we are very concerned. Has anyone else had an issue simular?