I wouldn't think you could have a court declare someone mentally incompetent without extensive evidence documenting it. I wonder if Sterling's wife and kids have been working on declaring the old man batty for some time now, and the chance of him gumming up the works on their $2 billion payday just escalated the issue.
Normally having one declared incompetent in court requires hearings and examinations by doctors. There may be language in the trust that would streamline this by out of court actions.