imbalanced torque
Lurker
For want of anyone more current and/or qualified, I suggest the torque difference would tend always to cause the left wing to drop.
The pilot flying will compensate for this with aileron and rudder, provided that he is not so aware of the real situation that he leads with the left engine all the time. The FDR tells us that he was indeed not balancing the engine output on this approach. The pilot flying will have had mildly crossed flying controls throughout the approach.
Pulling the power off, when that seems to leave the right engine still blowing air over the right wing, probably accentuated the tendency for the left wing to drop.
Earlier contributors have told us that the Metroliner was a brute to fly.
For me, the best explanation at present for the left wing drop turning into an unmanageable roll to the right has to include some stage of stall. The interim report mentions a continuous stall warning but does not give the critical values of angle of attack that the FDR might have recorded, nor any late change of elevator angle that might have worsened the situation.
Different aircraft, in or near the stall, might roll left or right with right aileron applied as was probably true here in the last few seconds.
More certainly, coarse right rudder (applied both to counter the excess thrust of the right engine and additionally to raise the left wing) will tend to reduce the effective angle of attack of the left wing and to delay its impending stall. The right wing stalls first and the aircraft rolls to the right - rapidly or even by way of a "flick".
Honest speculation, good enough till we get the FDR traces in the final report.