Sorry to join this late....I have experienced the same with Deez. He was totally fine as a pup, never had a problem and always socialised well with other dogs. However when he got to 1.5yrs, he started to become quite bolshy and would display the same signs you are talking about. Deez also had a bad experience, when a Lurcher turned on him (they'd been fine together) and ripped his ear so we weren't sure if this was related to this or he was just being a teenager!
We did end up castrating him, and going back to basics with his socialisation and he's now brilliant again with other dogs...but he does tend to be more tetchy if leaded. Although to be fair he does have a lot of little dogs on leads that lunge for him...but he's learning that we won't tolerate aggressive behaviour and that when we say leave we mean it.
I hope we're doing the right things for him in this case but if we're not I would always welcome any advice.
