This incarnation of the Secret Six springs from the Villains United mini-series, which was one of many that led up to DC's 2006 company-wide event, Infinite Crisis.
Catman, Deadshot, Ragdoll, Scandal and Knockout, (that's right, the Secret Six are only five), are now mercenaries for hire but they still have some baggage from their Villains United days in the form of the Secret Society of Super Villains. They became the Villains United when they spurned the offer of membership in the society and that was an insult that Dr. Psycho cannot abide. In an effort to forestall any revenge from the society, Scandal urges an offer of membership to one of Batman's rogues gallery. But as Catman says, "The cure may be worse than the disease."
The original Secret Six premiered in 1968 and had a secret agent motif à la Mission Impossible. This series has only the name in common with that title.