As the area of cross section of the hose (nozzle/outlet) of the tube is very narrow, there is a large force/pressure exerted by the water on the hose. The pressure/force is multiplied by the ratio of the areas of cross sections of the pipe at the inlet (of water) and at the outlet of water.
Water flows through a tube of a diameter around 5 cm to 10 cm. The flowing water has a force, kinetic energy, velocity associated with it. There is a motor that pumps the water through the pipe with a force/velocity. The velocity, force, pressure are directly related ie., proportional to one another. That is higher the velocity, higher pressure or higher force or higher kinetic energy or momentum.
There is a law in fluid motion, that the volume (or mass) of liquid that flows across the cross section is constant at any cross section. Since, the area of cross section at the outlet the hose is very small as compared to the inlet of water (at the tank), the velocity of water becomes very high. So the pressure and the force become very high.
Water applies force on the hose in the backward direction. The person has to hold the hose with a lot of force in the forward direction and with friction. So the fireman pushes the ground backwards, to get reaction force and friction to keep him stationary.