The announcement, in a Live Nation SEC filing today, offers no explanation for the resignation that it says was tendered Friday and was “effective immediately.” But it follows the surprise year-end resignation of Irving Azoff, who was the entertainment company’s executive chairman and is close to James Dolan. (Indeed, Azoff manages the guitar-playing cable exec’s blues band, JD And The Straight Shot.) Dolan was elected last year to a three-year term on the Live Nation board that was due to expire in 2015. And in 2011, Live Nation made special accommodations to bring aboard Dolan — who’s executive director of Madison Square Garden as well as CEO of Cablevision. Live Nation freed him from any obligation to present corporate opportunities to the company “except in limited circumstances.” It also said that, with a few exceptions, he had no obligation “to refrain from competing with, investing in competitors of, or negotiating on behalf of other entities against, Live Nation.” In another quirk, Dolan was not named to any board committee.