Synchronous generators have certain consumption dependent phase lead when they pump energy to the grid. Magnetization control affects only reactive power.
Burning fuel in motors, letting water to flow etc provide the torque which makes possible the generators lean forwards the right phase lead. If the consumption drops suddenly the torque is enough to speed up the generator, the frequency rises.
The control systems in different power plants have different ability to control the situation. There's no grid wide central command system which can reduce the torque in every generator synchronously with the consumption changes, central commands affect slower. Local control systems with their own delays try to perform the central command orders which are more like "produce this amount of power and the planned frequency and voltage are these"