In order for the water to change into steam there needs to be either lower pressure of higher temperature. When the water below the ground finally does boil and changes to steam its volume expands. This creates surrounding water to move upwards and consequently reduce pressure levels. At lower pressure and less depth some water flashes into steam and it continues to push more water upwards etc. Therefore, superheated water rises and carries with it the energy needed to make a geyser erupt.
Reduced pressure is what causes the gushing of a geyser.