This is definitely about the "Goddess Fall" incident.
For those that don't know:
Nikke were revered as the saviors of humanity, but after a catastrophic failure in a pivotal part of a war,
the human's perception of them changed from Heroes to Expendable tools.
They were treated as 3rd class citizens (at best),
and any human advocating for their rights were labeled as terrorists and subjected to persecution
200 Mass-produced Nikke decided to leave and build a home for themselves.
As they boarded an elevator to the surface, the government cut the power to stop them.
The elevator was 4km high when the power was cut and was now on a collision course.
The Nikke decided to save the humans below by sacrificing themselves by self-destructing and blowing up the elevator.
This is the reason why Nikke squads are 3 or less...