Imagine a grid supplied by a single coal-fired power plant. When demand rises the voltage drops and the frequency reduces, and the power plant has to start adding coal to the fire faster to get the voltage and frequency back to normal.

If the demand rises over the course of 10 minutes, you get a small drop in voltage and the plant ramps up gradually.

If the demand rises by the same amount over the course of 3 seconds, you'll get a much bigger voltage drop and the power plant has much less time to speed things up.

There's further complexity because real power grids are the size of nations and have many power plants. If several power plants notice the voltage dropping and increase their production, a few seconds later you might have too much power. Then they might respond by cutting their production, and a few seconds later you have too little power and so on.

> If the demand rises by the same amount over the course of 3 seconds, you'll get a much bigger voltage drop and the power plant has much less time to speed things up.

More than that, if the voltage drops too low, like from a sudden large power draw, you could get a brownout or even a blackout in severe cases. Voltage level is monitored and wild swings indicate instability which trips various safety systems.