Our 3 year old dog runs away and gets in fights with other dogs. We don’t understand this mysterious behavior.

We have a spayed female shepherd/lab mix named Sandy. We got her from these people at 1 year old, now she is 3. Every since we got our 7 month old husky puppy, Zoe, she has been acting weird. When we first got her, she ran away a lot, but was friendly to dogs. Now she runs away and picks fights. Her last fight we saw. We witnessed her in a yard and the dog that lived there sniffed her for a greeting and then Sandy just started fighting. She came home with a bitten ear and now she is starting to fight with Zoe, our puppy. Now we are afraid to keep her. What is going on and what can we do???