Incident: Jazz DH8C and CRJ1 at Montreal on Jun 22nd 2009, TCAS resolution

An Air Canada Jazz de Havilland Dash 8-300, registration C-FMDW performing flight QK-8739 from Bagotville,QC to Montreal Trudeau,QC (Canada), had originated in Bathurst,NB and diverted to Bagotville. The airplane was now flown to Montreal and on approach cleared to descend to 8000 feet.

An Air Canada Jazz Canadair CRJ-100, flight QK-7678 from Montreal Trudeau,QC (Canada) to Boston,MA (USA), had just taken off and had been cleared to climb to 7000 feet.

The crew of the Dash 8 had read back their clearance to descend to 8000 feet correctly, however continued to descend to 6000 feet. While the crew was queried by the air traffic controller, the traffic collision avoidance systems (TCAS) issued a resolution advisory ordering the Dash 8 to increase descent and the CRJ to climb. The CRJ climbed to 7300 feet temporarily.

The Canadian TSB reported, that the minimum separation between the aircraft was 1.4nm laterally and 500 feet vertically (required separation 3nm laterally or 1000 feet vertically).

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This piqued my interest since ZBF is my hometown. Anyone have any idea why the ZBF-YUL run would divert to YBG?