Our local free paper carried a story last week about an elderly gentleman and his 7 year old "perfect gentleman" golden retriever. They had been for a walk, mid-afternoon, and the gentleman was at his home and actually putting his key in the door, when out of nowhere came an off-lead staffy, who latched on to the retriever's leg. The owner arrived, swore at the dog, and eventually managed to detach her dog - leaving a wound down to the bone which narrowly missed an artery. Owner left without apology, elderly gentleman + dog very shaken up, dog required expensive vet attention. All this quite close to my house...:-(
Then, yesterday, at a hospital volunteer's "do" another volunteer told me she had been walking her 2 dogs off lead in Salcey Forest, when they were overtaken by a Weimeraner, who walked on ahead with its owner. Then, suddenly the Weimeraner turned back and attacked this lady's collie cross, tearing its side from the shoulder right down the chest. The vet said if it had been her other, thinner dog, it would probably have punctured a lung. The Weimeraner's owner was devastated, said it had never happened before, and has paid the vet's bill. So, the 2 owners' reactions were very different - but why do dogs suddenly launch these unprovoked attacks I know there's probably no answer to my question - but it's very scary, isn't it? :-(

Generally attacks are not completely unprovoked as such. 

There will either have been some exchange between the dogs that is often too subtle to notice, or as in the staffie case totally predatory action where the4 dog is treating the other as prey and recting merely on sight and the stimulus of the other dogs presence (same as dogs reacting to cats or other animals). 

In fact the Weims response may have had a similar reason, but more likely it ahd made eye contact of some sort.

Now these actions will ahve made sense to the dogs involved due to some fault in their makeup caused by training, esperience or nature.

I saw a Weim do this at a show he didn't cause any damage but only because he didn't get the chance as his owner pulled him off! :(   Two very well behaved flat coats being walked past on leash, not looking right or left, the Weim (waiting to go in the ring) looked behind and saw them and went berserk!  He really went for it :(  The Weim owner blamed the other dogs but they were no where near and didn't even look in his direction, and other dogs in between   From comments of the other Weim owners it wasn't unusual with that dog :( Definately owner not taking responsiblity there :(