A cyclist decelerating uphill has unbalanced forces acting on him. An unbalanced force acting on a body has a net force which is greater than zero. When a body is accelerating or decelerating, the forces are not balanced.
A body with a net force of zero has balanced forces acting on it.
Such body is moving with constant acceleration.
According to newton's first law of motion" a body will remain in state of rest or of uniform motion unless an external force acts on it".
The external force causes acceleration to change drastically.
