I think it's the restraint by the neck, that lunging against a chain on the neck has emotional impact. It triggers fight instinct. In a fight, they go for the neck, and the neck is where dogs will clamp even to discipline or enforce pecking order with each other. That's why leash training works, but it only makes the dog tense if the leash is held tight all the time. That they would be yanking against it when trying to do normal dog things, releases adrenaline, frustrates, angers or otherwise messes with their minds. When introducing two dominant dogs to each other on leash, if you hold the leashes tight, they are much more likely to fight than if you give them slack.
That was one of the main reasons I was nervous of feeding my neighbors two pit bulls. They have always been kept on chains....log chains. Moving them from their chains to their crates was scary.
Becky