What is actually happening
People call it warped rotors, and the mechanism is usually thickness variation rather than literal warping. Uneven pad deposits or heat cycling leave the rotor with high and low spots measured in thousandths of an inch. Every rotation, the pad grips slightly harder then slightly softer — and at 40 mph that is a rapid pulse you feel as a shake.
Steering wheel or seat?
Shake through the steering wheel points at the front rotors. Shake through the seat or brake pedal points at the rear. That one observation narrows the job considerably, and it is worth telling us which you feel when you book.
Why it came back after a brake job
If a shudder returned within a few thousand miles of new brakes, the usual causes are pads not bedded in properly, a rotor that was machined below spec instead of replaced, or wheel bolts tightened unevenly with a rattle gun rather than torqued to spec. All three are avoidable.
What we would do
Road test to confirm where you feel it. Wheels off, measure rotor thickness and runout against manufacturer specification, and check pad thickness and caliper slide condition. Photograph the measurements and send them to you. Replace only what is out of spec, torque to spec, and bed the pads in properly before returning the car.