That was exactly what I was thinking. I have read that sometimes when a dog is on leash they feel trapped, vulnerable. It can make them feel agressive. But sometimes it is the walker (as it was in my case) that causes the agression. If your dog was agressive in the beginning on a leash, and now when you walk him you're "prepared" for him to be agressive so you tense up and try to hold him close to you that can add to it as well. If you are tense and trying to keep him closer to you he may think that there must be danger. I did that to my dogs because they don't care for other dogs, so when we would walk by one I would shorten their leash... turns out I was making it way worse. So I was told to just keep walking by without changing speed or leash length and they would eventually get the picture....and they have. (sorta) They still don't like other dogs, but they are far less agressive when I act like nothing is wrong. (fortunatly thses two are only 30 lbs)
Vicki