Could be caused by "hot-spots" on the rotors - if you get the rotors really hot (track day type usage) and just park up and leave the car in one spot, then the rotor area under the pad cools vastly more slowly than the rest of the rotor, which work-hardens that part of the rotor friction-disc.
In ordinary motoring, it is not significant, but when the rotor gets hot, like when you really lean on the brakes, you get differential friction between the two segments of the rotor which translates as a steering shudder (eg: "grip a bit, grip a lot, grip a bit, grip a lot")
If this is the case, your rotors are going to be toast.
You might be able to see (on a cold rotor) a "bruise" on the rotor disc where it was "heat-treated".
RB
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