Preliminary report on the accident that claimed two lives indicates the tail boom separated from the helicopter after being struck by the main rotor blades.

In a teetering-head rotor system (such as installed on the Bell 206), if the helicopter is allowed to enter into a low-gravity flight regime ("unloaded"; "low-g"), it is possible for large cyclic control inputs to cause the blades to strike the boom.  In normal flight conditions it is not normal for a properly rigged main rotor to strike the boom.

Link to ATSB preliminary report:
http://www.atsb.gov.au/publications/investigation_reports/2011/aair/ao-2011-085.aspx