According to the indictment, part of how they were caught is that as part of laundering their proceeds, they tried to strongarm the payment processor Venmo, who had closed their accounts as part of automated fraud detection. Venmo was unhappy with being strongarmed, and sent a complaint to someone higher up at the agency. The agents then tried to suppress the complaint, and simultaneously retaliate against Venmo by trying to start an investigation. That attempted investigation pulled in the IRS, whose investigators thought a bunch of things looked suspicious, and dug up enough dirt to blow the whistle on the agents in this case.
So I guess in short, they pissed off both a payment company and the IRS.