Looks like the pilot just sat there and let it happen.

Even a small gust can induce ground resonance, or sometimes nothing at all, as depicted here:

http://www.youtube.com/watch?v=-LFLV47VAbI

This wasn't a 'ground resonance test gone wrong' despite the label, it was a run-out airframe at an Army maintenance depot, and someone had the idea to film the outcome for training. Full-scale heli pilots (may not include Brazilians) are cautioned against prolonged idling under power for just this reason.