In an electrical power plant, steam turbines are directly coupled to electrical generators, in a combination known as a turbogenerator. Turbogenerators have control systems on them that sense the frequency of the current that they are generating. When the load on the turbogenerator goes up, the speed (and frequency) goes down slightly due to conservation of energy considerations (e.g., work in must equal work out, assuming 100% efficiency), and the control system senses that change and increases steam to the turbine to compensate. Likewise, when the load on the turbogenerator goes down, the speed (and frequency) increases slightly, the control system senses that change, and it decreases the steam to the turbine to compensate.
The control system automatically takes care of these changes as long as it isn't constrained. At the point where the load on the system exceeds the ability of the control system to add more steam to the turbine, the excessive load on the system causes a "dip" in the frequency that cannot be corrected without some type of intervention. Such intervention could be adding additional generating capacity if it exists, rolling blackouts if all generating capacity is currently being used, etc. For the given plot, the 5-6 minute time interval of below-normal frequency implies that human operators had to respond by putting additional generating capacity online in order to meet increased demand.