Family dog attacks pet alpaca

We have a GSD who we have had for 2 years. He is loving and sweet and goofy. We have chickens and cats and alpacas on our little farm. He seems happy as can be, never chased any of them or showed any type of aggression. A few months ago he was “fighting” through the fence with the neighbor dog, which we discourage but they have a fence between them so we don’t worry. Our elderly chihuahua who is a bit too big for his britches went over to get in on the action and was bit by our own dog. We thought it was a fluke. A month after that, my husband got a Great Pyrenees puppy who, although we did everything we read to get him to be a guardian dog for the herd, has been aggressive, chasing the alpacas and even attacking chickens. Today my husband left the two dogs out and they brutally attacked our loving matriarch alpaca. The GP was vicious and attacked the neck and face and chest. The GSD got her in her rear and hind legs. Lots of punctures. We are at a loss. The GP has already been rehomed. But we love this GSD and don’t understand what is going on and why he would do this. Also we have two small children. Please help.