I've always understood that the airflow from the reverser buckets can be likened to an aerodynamic "barn door" shoved out into the airflow. By that I mean that the plume of air from the reversers is "seen" by the ambient airflow as a (semi) solid obstruction and hence creates drag which varies directly with airspeed.
The catastrophic destruction of the circulation around the wing of the Lauda B767 which had the uncommanded reverse at high altitude all those years ago would seem to support this simplistic theory.
OTOH, I may well be completely on the wrong tack - please add learned comments as necessary.