There was some pretty nasty weather in the region (snow/ice). My guess is that too much ice accumulated on the wings and the airplane stalled with no time to recover. I heard they were 2 miles out on approach for landing, if so, they'd have been probably too low to recover from a stall, which would have happened at higher airspeed if there was ice on the wings. And it would have come up on the pilots w/little to no warning. They'd be making a normal approach at normal approach speeds, then whammie, no lift and into the ground.
It is a very dark day for the Democratic Party. They definitely lost one of their best. If the politics play out and the Repos take charge of the Senate, Wellstone will flip over in his grave.