Not every dog trainer is up to dealing with this. It takes years of experience to understand. No one knows for sure. I believe genetics increasingly is the predominant cause. Now, what you have is a loaded weapon, literally.
Some dogs genetically have too much testosterone.
As we re busier and busier in our own lives, we sometimes don t socialize dogs early and positively as is necessary. More and more people are actually training their dogs to be aggressive - this is a major mistake. You re now losing control, the dog is no longer following your lead. Instead you ve got a dog making decisions on its own. This is exceedingly dangerous.