Looks and sounds as if the pilot had insufficient power available for a take off from a confined area (probably hot) towards a high vertical obstruction and overpitched the blades - hence the reduction in rotor rpm that can be heard. Think that type has a audio warning for engine failure, but not for low rpm (might be wrong). Therfore both engines running but unable to provide enough power would give the loss of rpm without any warning tone as heard (or not) on the video.
Believe that there is another thread from when the accident happened.