If it was "swirling around" as one eyewitness described it, it's an antitorque failure, essentially certainly. What actually caused the loss of antitorque -- i.e. what failed (conx to pedals, linkage, clipped a tail rotor blade, threw a tail rotor blade, etc.) is another question. The spinning stops instantly the moment you chop the power, but at the high power setting undoubtedly used to get off the pad, you're going to be 360 degrees around by the time the brain realizes the failure and you react; sometimes, the brain doesn't react even in experienced pilots before the thing tips over and thrashes itself. Sometimes, the brain struggles against the pilot's training because of the context, seeking an alternative to a low altitude autorotation, in this case into the water, and the thing can be really out of control before you flick your wrist and cut it.
For those unfamiliar with this heliport, there are several side-by-side lanes where the helicopters land, nose facing the wall of the heliport, perpendicular to the river. A typical departure is to pick up to a hover, back out of the slot the way you came in and, once over the river, 180-degree pedal turn and accelerate at constant altitude followed by a climb once you have developed forward speed. Of course, YMMV. I have seen it done conservatively and I have seen it done showy (i.e. high-ish hover followed by nose-high, tail deep down backing, fast pedal turn to nose-down-tail-high and dramatic acceleration). The opportunity to clip something certainly exists in that regime...not to say that this was what the guy was doing; you can clip the pier on the pedal turn in a relatively-flat departure as well.
Interestingly, though, if you are going to have a power failure (or intentional power chop) at low forward speed, the Jet Ranger is a good friend. Because of the mass of the rotor system, there is a lot of kinetic energy available for you to "spend" cushioning the landing or maybe even trying to get back to land. (As an illustration, I have seen a very-highly-skilled instructor bring the rotors of a Jet Ranger to operating speed on the pad, cut the engine, pick it up to a low hover, do a 360-degree pedal turn, and put it back down. It's a very nuanced maneuver, but possible, and testament to the design. Try that in a Robinson.)
It will be interesting to see if there is any security or other video of the actual incident, and what the pilot has to say.