Why does the frequency of power generation change when there is a change of load on the power grid?
why does the frequency of power generation change when there is a
  change of load on the power grid?


It's a classic control system problem and nothing to do with any background stated. With finite gain error amplifiers, a finite error is required to drive the system into near-lock. The error amplitude is the amount of frequency deviation from 50 Hz being currently produced - that error drives the generators to speed up or slow down. Under transient conditions this is exaggerated because there is lag in the system and things that weigh hundreds of tonnes take time to catch up to speed. 

Without a control system, a generator would make a very poor generator in that as you add load it will slow down and, to speed it back up you need to apply more mechanical effort (more coal, more gas, more steam or more water through the hydro turbines).

Keeping the voltage constant (at source) is easier because you can use field winding control to increase or decrease the generated voltage but, frequency depends on the spinning rate of the generator.