Eh...heheh... *cough*

Well...

The planet of Mechaius used to be a totally normal, though alive planet (this "alive planet" thing should sound familiar to FF fans. )
That was the situation until a virus destroyed its inner core, practically its heart.
There were already humans (or whatever. Just intelligent creatures. ) already inhabited the planet, and they used technology in order to keep the planet "working".
As their technology kept getting more and more advanced, they began to turn themselves into cyborgs, to gain immortality, as Onag already said.
This continued to a point at that a part of the population only consisted of the most important part of their brains inside heavy metal suits.
These began acting like the Borgs, warring those who still had parts of their own bodies to gain power over the planet.

This could also be caused by the virus.. that could give an extra quest, or several even.