Before confrontation, dogs usually give warnings by growling and with body language. However, it can occur without any long and apparent announcement.
Some of the reasons for confrontation:
1. A result of meeting two dominant dogs. It occurs due to urge to impose oneself as a leader while the other dog that interprets dominant signals does not accept it. Usually it is followed by a dog dance which is one of body language acts which purpose is to show the attitude.
2. Because of the fear or necessity to defend itself.
3. As a product of behavioural problems. In this case the dog attacks for no obvious reason.
4. It can happen that unstable dog threatens the stable, dominant one. This usually ends with avoidance of a conflict, since dominant dog can sense the instability, but if attacked it will return the favour.