Why does Vmca decrease with increasing altitude?
For airplanes with normally aspirated engines or turbocharged engine above their critical altitude, the engines produce less power as altitude increases as atmospheric air density decreases as altitude increases.  As power decreases, the operative engine requires less rudder authority to maintain directional stability due to the reduction in thrust, which allows the airplane to maintain directional authority at lower indicated airspeeds.  Therefore Vmca decreases as altitude increases.