Thread: Stock-proofing????

Ok i need help/advice please

Sorry for a long story but il start from the begining so you all understand my situation:-)

Ok so 3years ago my sister and her husband adopted a beutiful ridgeback cross lab bitch named Shumba in manakau,she is such a clever dog and is obedient,she sits,stays,rolls over,begs and even hi fives!
Me and my gf had to adopt her last year as my sister and husband moved to bali for work,a year on and she hasnt dis-obeyed us(apart from the odd hole in the garden),she is walked almost every day and is happy and healthy,
So at christmas just gone we took her down to the inlaws farm and she was tied up(just for the day) as we wernt sure if she was safe with stock,(tied up next to sheep paddock)but sure enough she chewed throught the leash and came running to us for attention..all was good
So two weeks ago me and my gf went to bali for a holiday and gave shumba to my father who has looked after her before and takes her for walks along the beach and river bank(with stock nearby) with no lead and shumba has had no interest in stock before,she comes when she is called without a problem,,,
But this time(in the weekend just gone) there were other dogs around and they started playing as usual(she loves other dogs) and according th my dad she suddenly out of the blue took off and chased a sheep around and attacked it!!!(dad said sheep will be fine) but why has she done this all of a sudden????
Is it a pack instinct?even though other dogs were not interested?or was it a chase for fun factor??HELP??
It is completly out of her nature for her to do this...any advice/suggestions would be much appreciated
:-) Thanks guys