There are probably tons of reasons why your baby may be snapping, not the least of which is they have decided to be the boss.
I have read many postings on correcting aggressive behavior, and I am sure if you do a search you will find a boatload of helpful postings. It takes patience and repetition, just like all the training. I can't help you any more than point you in that direction, as I have not had to personally deal with this.
However, I was reading an old post last night and the person writing found out that their dog had been injured by a neighbor without their knowledge around the same time that the dog went through a radical personality change. Is it possible your little friend had an injury or an illness of some type? The injury I am referring to was spinal in nature, and the pain had made the dog aggressive.
Good luck!