
I have seen few dogs with problems but I would say many of these are caused by inexperienced owners thinking they will have a gentle teddybear without putting any work in when the dog is young, few months later have 60kg of dog to handle and teach about world which is not easy!
Most other "aggression issues" I have come across have actually been because of lack of confidence(both owner&dog) so often loads of noice but no action.
My first one was like this, would bark and scream on the lead but if let to go and say hello she would act in very unconfident manner.
Problem was not many people would let her say hello after she barked, which I can totally understand but makes it difficult to do what they need the most which is socialise!
She did grow out of it when she matured(+loads of work) and was later used as a dog other non confident dogs would get to greet at the training club due to her calm non-pushy mannerism...
Also, Leos can be pretty high energy especially when young and I do not think all owners are expecting this.
Just like Susie says, many seem to go through a period when they kind of reverse behaviour vice and try their limits when around two years old...
Often this is a first time the dog that has been easy dog suddenly has issues and this can throw the owner a little.
Most "barkers" fit to this age period and many will mature to be "nice" dogs when the brain grows a little!
They are generally pretty sensitive breed and need to be guided to behave correctly from young age...firm and fair handling is needed.
I remember the puppy guide we had with our first one(+15 years ago) having a sentence that said something like: Leo's need to be handled with firm but gentle attitude, too hard handling will shut the dog down and will get you nowhere. I still think this is so true...