The crew stated they were in a heated discussion over airline policy and they lost situational awareness.
Here's the track of the merger discussion:
FlightAware > Northwest Airlines Inc. #188 > 21-Oct-2009 > KSAN-KMSP
Good thing they are in the union, else they would be in big trouble.
I try to avoid these discussions in flight myself but some folks seem to want to bait you until you snap back at them. Some fisticuffs have already taken place at venues like Yankees bar in NRT from what I hear.