I have two dogs that have been with each other for years and get along just fine at home. But recently whenever we take them on a walk if they see another dog one of them gets really aggressive and attacks the other. She didn't used to do this but she just started doing out of nowhere one day a few months ago. Why is she doing this and how can I get her to stop without getting bitten?
The dog attack the other is a female Ryukyu Inu that is 5 years old. The other is a male Lab/Chow mix that is 8 years old.

This sounds like a case of displaced aggression.
Pretty simply put, when your dog is overly aroused by the presence of other dogs and cannot either get to them or away from them, that frustration is taken out the nearest thing, the leash, his walking mate or in some cases the owner. This can make for a pretty dangerous situation.
My suggestion would be to find a behaviorist in your area that can asses your particular dog, since this behavior has the potential for injury, not only to your other dog but to you also.
Other suggestions are to keep your dogs out of areas where they encounter other dogs, muzzles, and head halters.
But the only way to come close to a "fix" is through behavior modification.
By the way are they spayed and neutered?