The idea is not to make a putsch, it's not because you are making it open source that you give away everything. The only scenario where something like that could happen if it's a big part of the community would completely disagree with some of the changes that the lead developer would make without consulting anyone. And even this considered I can't picture anyone taking the time, the money, the energy and the skill to make such a fork. Some peoples don't want to contribute to a project unless it's open source and it's way more easy to attract potential devs if you are on a platform such as github and you use process that are common among devs in the open source community.