figure it out why it happened this way

I believe that you are telling the truth of what has happened, the question is why it happened. It's apparent to me that you are driving the project and you know the best of the project. I would say that your current boss picked the wrong guy as the project leader. He doesn't know how to handle the situation of being promoted to lead formor coworkers. He is not self-confidient that's why he sees you as a threat and cut your exposure. In normal situation, his project will fail miserablely and he will face the consequence of not being able to lead. But that will be his problem.
However, the outcome says otherwise. It looks that he got things delivered. If the situation continues a little longer, people will naturally think that he is a qulified leader. So something doesn't add up. A wrong choice of a project leader plus a project leader who doesn't know how to do the project and doesn't know how to run a group can result to a successful project? You are doing something that is against your best interest. The project needs to fail to prove your point and. Make sure that you let other people know that you completed your fair part of shares so you will not be blamed for the failure.
Another important thing which I also mentioned in one of my posts: you need to update your A's manager but don't be seen by your A. Openly talking against the project leader doesn't help at all. The key is to constantly update A's manager. For example, if you have a good idea of what to do, don't share with A. Instead, let A does thing his way then later suggest the better approach to your manager. There are a lot things you can do to acchive the goal, you just need to think.