This group of Borg were separated from the Borg Collective in 2373 when a Borg cube was damaged by an electrokinetic storm while traveling through the Nekrit Expanse. The encounter with the storm resulted in the separation of the Borg drones from the Collective, allowing the survivors to regain their individuality and settle on an isolated planet.
Unfortunately, the sudden return of their individuality resulted in the many survivors fighting one another as hatred, as well as other war-like characteristics, consumed the survivors. One faction urged for peaceful co-existence and was to become known as the Borg Cooperative. They attempted to recreate the subspace link to form a sub-Collective and end the fighting, but faced constant attacks from the other Borg survivor factions and eventually sent a distress call which was answered by Commander Chakotay of the USS Voyager.
However, Chakotay's shuttle was damaged and he injured. The Cooperative cared for him and used a Borg linking technique to heal his wounds. They used this tactic to force Chakotay into re-establishing the Borg link by using the technology on the damaged cube that remained in space. This re-established a sub-Collective and ended the fighting with the resulting creation of a faction of peaceful Borg. Once established, they destroyed the Cube and were determined to live on their own planet in peace. (VOY episode: "Unity")
In an alternate timeline where Voyager was forced to stay in the Delta Quadrant and stay in the Vostigye Union, the Borg Cooperative was destroyed by Species 8472 during their war against the Collective. (VOY - Myriad Universes novel: Places of Exile)