Why did our Husky attacked our small dog?

My brother has a 4 year old, 50 lb female Husky. The other day, with NO warning, she attacked my 9 lb 10 year old Shih Tzu. All my dog did was walk by her on the way out of the room. She grabbed ahold of her by the neck and jerked her until my Mom pried her off. My dog has several bite marks and a slipped Cervical Disk in her neck. The husky used to bother my dog years ago, but not this severly. She almost killed her this time and I don't know why. The worst part is, the Husky doesn't think she did anything wrong. She didn't submit or even look guilty when she was yelled at. I am trying to keep them separated, but it is a small house. My dog has a heart condition and is VERY small. i am so worried about this...any suggestions on the cause or how to fix this?