All brake work gets a test drive. They should have noticed if a shimmy was still there. If the brake work was done a few months ago, they may have used Chinese rotors. There's nothing wrong with them but when we make parts here from cast iron, we let them "age" 90 days before they get their final machining. Chinese rotors age on the car and often warp in a few weeks. Once they get another touch-up, they're fine. Any reputable shop will do that for free if they did the original repairs.
Debris and scale can also get stuck between some rotors and hubs and cause a shimmy. Broken tire belts will do that too but you'll also feel that when you're not braking.