Replace the coupler to correct the problem.
This situation is usually associated with the reel raise circuit. Lowering the reel down completely should relieve pressure sufficiently to allow the hose to be disconnected.
CAUTION: Avoid High-Pressure Fluids. Escaping fluid under pressure can penetrate the skin causing serious injury. Avoid the hazard by relieving pressure before disconnecting hydraulic or other lines. Tighten all connections before applying pressure.