I did try and keep the background somewhat in line with the Star Union story but didn't go into too much detail. I was going to make a campaign at a much later stage but with the amount work still I'm adding and plans to collaborate on another project i wont have time.
Long story
"Here is the rough premises on how they initially formed, when the void storms went up a large section of outer colonies of frontier worlds like 'space islands 'of the Star Union were isolated together, mostly made up of small industrial companies, agriculture and your general backwater rejects of Star Union officials and small syndicate houses sent there from whatever disgrace reasons to oversee the operations.
Where the story begins is one of the local planet magistrates contacts our 'hero' leading one of the larger mercenary companies in this sector, with no other work going the hero agrees to meet, the magistrate tells the the mercenary of a disturbing report of how some of the other planets communications had gone dark and want the mercenary and their men/woman to investigate. A seven samurai scenario.
The story unfolds of a large host of hoppers have been engulfing planet after planet. And our hero leading the mercenaries initially help in evacuating planets. With more and more planets needing assistance and new breeds of hoppers start to appear the mercenaries start gaining more support and developing defensive strategies in repelling large hosts, as well as guerilla tactics to destroy nests behind enemy lines. In the end the mercenaries and our hero give no more ground the mass swarm expansion is halted with the killing of a large cunning hopper directing the attacks.
In the end when the void storm clears the sector is at a stale mate , the mercenaries have become the official military force 'Colonial Guard' for the whole sector of planets made up of all the races, however the cities are suffering from overpopulation. The reasons for the Colonials to be dispatched to the heart of the Star Union are varied, most been ordered to relieve the overpopulation and settle on new permanent worlds, and continue to offer the same service while other Colonial Commanders simply may have deserted for opportunities, or had been bribed from someone of note to work for." Source - Some drunk ramblings of a Colonial Marine
So that's one theory but it may not be true, basically its up to the players on the details of how and why. So to your question, mostly for the game play mechanics but had put thought into the lore.