I'm as critical as activist judges as anybody is (meaning I dislike them until they rule the way I want on something), but I don't see how this judge was activist.
Broadly speaking, he needed to determine whether Shelley Sterling acted properly in managing the Sterling family trust. He had to determine whether Mrs. Sterling had the authority to act, which involved the dementia diagnoses. Once he found that she had the authority, he had to determine whether she acted properly, which is where the valuation testimony was relevant.
From my review of the trial, it seems like it was a slam dunk case that Shelley Sterling had the authority to sell the team, and that the sale was in the best interests of the trust. The only legal question I'm unsure about is the "1310(b)" finding, which makes the probate action non-appealable because an appeal could be harmful to the team or the trust. The statute appears to clearly allow the judge's action, but I don't know anything about the way case law has evolved on that issue.