We know that the entire grid is at the exact same frequency (50Hz or 60Hz). This is because the grid is made up of synchronous machines. The difference is that of the phase angles. If you go back to the fundamentals of synchronous machines, you will see how important the frequency is to its operation. In fact, power transfer between two nodes depends largely on the angular difference between the nodes.
Load is inversely proportional to frequency. When load increases, that is current drawn from source like generator increases, voltage drops, as EMF equation states frequency will decrease. And as electrical load on generator increases the speed of generator goes down as Ns= 120f / p, Generator will not attend the required speed to generate, causes loss of synchronization, so there is should be variations in frequency within that Load can be managed to maintain the system in normal condition. As we know that for synchronization voltage, frequency, phase sequence must be equal.
First of all remember that frequency (50Hz or 60Hz) is related to speed which is a mechanical property. So when you have 2 generators trying to synch together you are matching voltage, phase angle, frequency (speed), and phase configuration (rotation) is the same. Connecting two generators not running at the same speed can have consequences where the faster running generator pulls the slower generator to match its speed. This can cause structural damage to the generator (think of two gears meshing together that's not running at the same speed.
Electrically this can cause power surges (i.e. a departure from equilibrium) throughout the electric power grid. I don't know how to explain it precisely, but if a generator goes out of step, the internal forces of attraction break. Remember that the torque in the generator depends on the angular difference. Not a completely clear explanation, I know, but we can clarify as comments go back and forth.