The most common reason is one or more of the rotors is warped. If you also feel it in the steering wheel, the check the front first. More uncommon is that wheel bearings are loose, front tire pressure is too low, or you lost a wheel weight.
First consider if everything was fine until you changed something. If you are checking rotors for run-out, they should be removed from the vehicle and measures independently. For a quick check on the front rotors, temporarily tighten the spindle nut to avoid any extra play. This works but isn’t entirely accurate. You will typically feel pulsations at about .007″ of run-out. The specs are the same for the rear, but these should really be removed. The rotor is attached to the axle face near the axle bearing. The far end of the axle are splines rotating within the spool. There is clearance in the fit of the splines which is amplified by the axle length. You only want to resurface rotors that have run-out. Machining them unnecessarily reduces their life and can make them more prone to warpage due to loss of mass.