Sometimes powerful elected officials and controlling parties can work at cross purposes here. Barney Frank, for instance, may want the lines drawn to bring in enough lefties to give him a safe margin. Fine. That means ALL of the surrounding districts have fewer lefties, making them more competitive. In MASS, that usually doesn’t make a difference. This year it might.
This REALLY plays out well in southern districts where Republicans happily draw districts that are 80-90% minority, to ... you know, make sure they get their own seat ... at the cost of all the surrounding districts. The black politicans elected in those heavily minority districts are all too happy to play along.