Reading between the lines here, I perceive that there were personality conflicts that lead to this decision. But, this reason seems odd.
Open source does not mean you have to incorporate source code of others, it just means your source code is available. You can work away as an elite team and then release your product with source code; you would still be open source.
Even if someone else comes along, and says 'Hey, I want the project to go this way!', you can easily split up, just keep rights to the name and they can be the 'other project'. You might even find you can incorporate some of their features later.
I can understand people wishing to close source a project if they later intend to use the project as a base for consulting on for money, but to close source for ego reasons, just seems odd.
If you open source a project, then go closed source, but state your software will still be free, then why should anyone not also believe that you might later change your mind and charge for the software? (just release another version give it a new name etc.).
If a person wants to make money from software there is nothing wrong with this, but why obscure the issue?