one of the few things that tenured professors can be fired for is misconduct.
You can be fired for misconduct, but posting anonymous criticisms on a public web page would not rise to the level of misconduct that justifies revoking tenure. I mean, we might all think that the guy behaved badly, but that doesn't make it misconduct as the concept has been applied in universities. It pretty much has to be gross violation of your responsibilities to students, or faking data (in the sciences) or plagiarism (in the humanities). Even these things rarely result in dismissal. Spreading rumors about your colleagues? Standard. Finding ways to take advantage of new technology in doing so? The academic way.