I have a Beagle and a larger mixed breed. For no reason the larger dog attacked the Beagle. Will this happen again?

I was hoping maybe you could help me with a few questions regarding our two family dogs. We have a six year old female Beagle, and a Three and a half year old mixed breed female who is about 85lbs. These two dogs have lived together for all of the larger dog's life and never had any kind of fight or altercation, until yesterday. For no apparent reason the mixed breed attacked the Beagle leaving four large puncture wounds on both sides of her neck. The Beagle was treated at the emergency vet and released. She was heavily sedated and slept until this morning. After only a few hours of her waking up she was attacked again (viciously). My husband and I finally pried the larger dog's mouth off of her and rushed her to the vet again. This time she has to stay for a few days because the damage was much worse. My husband and I were both bitten during this attack and I am afraid to bring the beagle home with the other dog here. My questions are:

1. What would make her turn on her all of a sudden? (they are great friends)


2. Would having them both spayed possibly prevent this from happening again?


3. Should I worry that this will happen again and find a new home for one of the dogs?

Thank You,
Jennifer