I recently learned about slab rollback, specifically of the Juan de Fuca plate subducting underneath the North American plate. From what I understand, the subducting plate over time begins descending at a steeper rate, "pulling" the trench and the above converging plate back (towards the direction of the subducting plate). What makes the subducting plate begin to sink at a steeper incline? My guess is an increase in the density of the plate allows it to descend steeper, but I don't know of any evidence to prove that.