The ELI5 version is you're arm-wrestling someone and they suddenly let their arm go limp, so your arm slams down on the table since you can't react that fast.
To build on this analogy, tug of war fits a bit better. Nothing dramatic happens if one person let go, but if half of one team just let loose at the same time without communication, bad things happen.
I think an even better example is what happened to the Tacoma Bridge. You have an oscillating frequency and phase that needs to be precisely tuned in order for everything to work well. If something is out of sync wrong, you can end up with higher peaks, lower valleys, or flatter locations which can ultimately cause catastrophic failure.
In the Tacoma example, the input frequency continued to add onto the bridges motion which ultimately caused it to destroy itself. In an electric grid, a misaligned phase can cause excessive spikes (imagine 480V when you expect 240V) or the generators to ultimately burn themselves out because 2 generators are fighting with each other, one trying to raise the voltage while another is trying to decrease the voltage. The really tricky thing is that load (particularly inductive or capacitive loads) look almost exactly like an unaligned generator.
Reminded me of this fun video of a guy spinning up a water turbine generator and getting it synchronized with to the grid the old-style needle phase meter, then connecting it successfully:
I interpreted from that EU grid post mortem that individual generators are coordinated using out of band comms channels that aren’t the power grid itself. Am I mistaken that they do this? Is it done there, but absent here?
Communication that detects the release and travels as close as possible to the speed of the natural signal in the rope, and is robust enough to recover without losing stability if the other team grabs the rope again while your "let go" signal is mid-flight.
One way to dampen this is to put a really strong guy on each side, with instructions to never let go of the rope. These are the flywheels of the grid.