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You are increasing the load until instability occurs. The transmission system becomes more and more stressed and at one point there is loss of synchronism between parts of the system. I.e., one or more generators goes out-of-step with the rest.

Plot the rotor angles of the machines in order to see which part of the system that is going out-of-step. When out-of-step occurs, there will be 0 voltage at points in the system, most likely inside transmission lines (or inside transformers).

Plot all bus voltages to see where the voltage becomes lowest during the out of step. There is a voltage dip each time there is an out-of-step going on in the system. Here you see repeated out-of-steps, causing the oscillating voltage.

You can repeat the simulation with dynamic options "Scan out of step conditions - Monitor and trip" enabled. The branches and transformers involving the out-of-step will then be tripped automatically and you can easily see where the out-of-step occurs. There will then be a system split, leading to two separate AC-systems.