All generators have lost synchronism. The redistribution of load caused the remaining stable generators to exceed their transfer limits, leading to a cascading failure.
When the entire grid collapses, operators cannot just flip a switch. They must perform a Black Start. Small isolated diesel or hydro generators are used to slowly power up the main plants. Before connecting any two plants, their AC voltage frequencies (speeds) and phases must be perfectly matched, or they will violently reject each other and trip again.
Think of the Rotor Angle as a magnetic rubber band connecting the physical generator to the power grid. As the grid demands more power, the rubber band stretches (angle increases). If the angle exceeds ~90°, the rubber band snaps! The generator loses its magnetic grip, spins out of control, and disconnects to save itself.
The total grid load is split equally among active generators. However, each plant has a different maximum capacity. Because G3 (Nuclear) is the smallest in this sim, pulling its share of the load stretches its "magnetic rubber band" much further than the massive G1 (Hydro) plant.
If you cut a transmission line or cause a power surge, that generator might trip and disconnect. The grid still demands the same total power, so the lost generator's load is instantly shoved onto the surviving generators. If the survivors can't handle the sudden weight, their rubber bands snap too, causing a domino effect.
Even though the grid demands the same amount of power (load) from each active generator, they don't have the same maximum capacity (Pmax).
Rotor Angle (δ) = arcsin( Load / Max Capacity )
Total power requested by the city. Split equally among active generators.