What is the effect on power grid when utilization goes down some threshold?
If you are pushing a cart up a hill, you need to push harder if the cart is full of bricks and not as hard if it is empty. If the load falls off and you keep pushing with the same effort, the cart will go faster.

With electric generators, a certain amount of effort is required to turn the generator. If the load is shut off, the same effort will make it go faster. If it goes faster, the frequency and voltage will increase. Generators have regulators that sense the load and automatically reduce the effort if the load decreases. Reducing the effort of the engines, turbines etc. that turn large generators takes some time and care. That is the cause of concern. The power grid equipment and operators should be able to handle demand variation without too much concern, but they apparently are not universally trusted.