Wow. Just wow. What a moron.
Having been an Army Reservist (and Active Duty before that), this must have been the payroll system for Civilians that work at Army Reserve centers for the Army itself and maybe Unit Administrators when they're not called back to duty and militarized under Title 10 orders.
And given most UAs are retired First Sergeants and Sergeants Major, what a stupid target to fuck with. I would not want every UA in the country hot after your ass for making them miss a paycheck. At the same time, these people can likely handle missing a paycheck whereas a Private up to Specialist/Corporal really can't unless they're living in the barracks and most of their pay is disposable income.
Military pay for the actual servicemembers in an Army Reserve or Regular Army unit is (mis)handled by the Defense Financial and Accounting Service, which is an even dumber target to fuck with. The easiest way to wind up in prison if you have anything to do with the Department of Defense is to mess with Uncle Sam's money. They've even tossed the first commander of SEAL Team Six/DEVGRU into prison for misappropriating funds.
That being said, I'm amazed that the Clowns in Disguise (CID for the uninitiated, they're usually pretty inept) figured it out, and it didn't take the 902d Counterintelligence and NSA/CSS to do it, unless it did and the CID's just getting the public credit for it. Which could be the case.