Why is my dog attacked when he's on the lead?

This is very very common and the cause is often the inability for the on leash dog to greet oncoming dogs politely because we prevent them from turning away, approaching in an arc, and coming in from behind. When we prevent that enough (particularly when coupled with allowing pulling on the lead), our dogs assume it's ok to put as much effort as they possibly can in getting to strange dogs head on.

Intolerant and socially able dogs will attack these dogs for their impudence - more often than not, without any real physical damage, but to the uninitiated, it can look and sound savage.

It's not at all unreasonable of these dogs to attack our leashed dog who has learned to be rude.

We do however, have to teach our dogs to approach properly and politely. often all we need to do is allow them space to do so - ie let them go. But they will need encouragement when they do the right thing - they will get duffed up a few times by strange dogs before the learning is cemented.

Walking with intolerant, but not aggressive dogs is a good start. Let everyone go and monitor behaviour. You can be ready to interrupt if everyone gets too excited, but not so soon that learning doesn't occur.

Ideally, dogs will just jog on by strange dogs after a glance to see if interaction is on the cards, if it isn't, or your dog doesn't want to engage, he should offer some 'no thanks' signals - turning his head away, turning his bottom towards the stranger, even a tiny nose flick to the outside should be read as 'no thanks', but often is missed in under socialised dogs. Hopefully, small signals will always be offered before big bollockings, but if small signals are ignored, or the approach is rude, expect trouble. If you turn away and offer your bottom to the approaching dog, your dog should also do the same, which may diffuse the trouble - you can get super interested in the floor off to the side if you like - if your dog joins in, you'll likely diffuse anything that might have been about to occur.