How and why are NFL coaches so fluid with their jobs? By fluid I mean in a league that seems to be all about specialization, coaches seem to move around positions pretty fluidly.  For example, Hue Jackson with the Bengals went from DB coach to RB coach to OC and Ben McAdoo with the Packers went from TE coach to QB coach to OC and Juan Castillo jumped from OL coach to DC with the Eagles.  