my speculation is, that what made them to declare an emergency, to make 180 turn and follow the order to land in Minsk, was threat from the KGB agents on board.
All evidece points to no agent on board (see in particular Belingacat research on the subject) - the people who left where merely some passengers travelling to Minsk in the first place.
In any case no-one has mentioned any disturbance in the cabin whatsoever (execept for some "rumors").
The pilots were tricked into landing in Minsk, which they did on their own accord. End of the story. A pretty crude con but not much to do with a highjacking at gun point.