Melted polar ice water makes the oceans deeper on average. The extra water on top of the oceanic plates makes them sink relative to the continental plates. This relative vertical displacement will facilitate the subduction of any oceanic plates that are currently stuck (like the one off the coast of Oregon, which is about 500 years over due for a massive jerk that will set off the long anticipated "Big One").
It is like car bumpers: if the bumper of a lighter, pushed car is higher than the bumper of a heavier, pushing car, the pushed car bumper will eventually ride up onto the bumper of the pushing car when the weight disparity becomes great enough. The greater the disparity, the more pronounced the effect.