Subraces are usually a powerful, but very small group within one of the main races in Drox Operative. These subraces won't exist at all at the beginning of a new game, but can come about in various ways. Some just happen randomly due to evolution, some are created by their parent race in response to dangers, and some are just accidents. Because they are individually more powerful than an ordinary person of the race in question, they are often used for their talents and abused because they are different, but they are also generally feared. So it's not a huge surprise that when given a chance they rebel and are willing to die for their freedom.
Their rebellions can be very nasty affairs for both sides. The government is likely to quell the rebellion with deadly force and the subrace rebels will likely use their talents to gain their freedom. This of course is where the player comes in. You actually have a choice, which side do you help out? Do you help the oppressive government that you already have a relationship with or do you help out the dangerous rebels?
If the rebellion is successful, the subrace will become a fully functioning new race in your sector of the galaxy. This means they will start building ships, have a functioning economy, scout and colonize planets, and start diplomatic relationships with the other races. This will definitely be disruptive to the sector. There is now a new force to deal with, they are powerful, and they tend to be pissed off due to their mistreatment. This also means if they are ultimately successful, you better hope you backed them when they asked for help.