One possible answer: its owners' fear. As a dog grows up and loses its reassuring puppy appearance, the owners can become afraid of its attempts to impose itself on its own kind. Interrupting these interactions, telling the dog off or preventing them can however have negative consequences.
I started dealing with aggressiveness during a rehabilitation of ex-fighting pit bulls project. I felt myself in charge to protect people working with me on the project, and to understand if and how those dogs could be given to families and introduced in the community. I witnessed an assault - this pit bull had attacked the trainer during the assessment - and I was shocked. I had no knowledge about those dogs, therefore, to achieve a better knowledge of those dogs themselves and the breed, I chose to use a test assessment. That's how I began to study dog aggressiveness.