Jackson came to Reddit with an offer: donate at least $70,000 to Alzheimer's Association, and he would select a monologue to read from all their submissions. The donations ran so high ($130,000!) that he decided to read two monologues.
In the first monologue, he cancels the monologue to announce he's giving up acting to become a vigilante crime-fighter.
Which was faked. The "vigilante announcement" was just the chosen monologue. Trolling successful.