Why would our dog suddenly attack its partner dog. It happends usually when there is somekind of excitement while they are outside

Were the fighting over a food source that you could see?

Tick

Ages and sexes please?

Are they spayed and/or neutered?

Could be a struggle for dominance. Could be play. Could be territory.

Assert your leadership so that they don't injure one another. Usually all it takes to stop a "disagreement" is a distraction when they become too intense. A whistle and a garden hose have always worked for me, whistle first, then water if they try again.