FBinNY 

The shudder is a harmonic effect similar to how you can make a wine glass sing by rubbing your finger around the rim at the right speed an pressure.

It needs two conditions to happen.

1- a variation in traction force, aka grip/slip which changes the loads on the various parts.
2- something that moves in response to the braking force, and can move to a stop, then slip back to the otiginal position. The movement can be at the headset, play in the arms, flex in tha arms that allow twisting with the forward motion of the rims, or even flex in the fork blades which allow the whole fork to flex back, then release and spring forward.

Many carbon forks are a bit flexy and can cause shudder even if all is dialed in, but save blaming the fork for last.

TEST for and locate any play, by applying the brake as you might when it shudders, and pushing the bike forward and down (to prevent the wheel from skidding). If you get any feeling of grip slip, or movement to a stop, find where it is by putting your fingers on the lower headset, the brake arms, and also feeling for twisting at the shoe.

One common cause of shudder is brake arm twist, especially with new shoes. As the rim is moving forward it wants to take the shoes with it, so they exert a twisting force on the brake arms. At higher braking force, the arm forces the shoe flat, but in the mid range, the shoe can twist momentarilly lifting the toe, and digging the heel in with an effect like new chalk on a chalkboard. We toe brakes in slightly to offset the twist, and prevent the heel from biting.

Ironically, grippier shoes are more prone to shuddering because the forward force at the rim;'s surface is greater realtive to the squeezing force keeping the shoe flat. But I'm not suggesting that you oil the shoes to quiet them.

Generally (not always) shuddering improves as the shoes brak in , because the heel corner breaks down. If they don't improve, or you don't want to wait, you can reduce shuddering by filing s "ski tip" ramp on the back corner of the shoe. You want to go about 1/16" deep and angle in over 1/4" or so. This eliminates the heel's ability to dig in, but will also reduce the shoe's effectiveness in the rain, so consider it a last resort solution.

BTW- it you have a carbon fork, and all parts are solidly tight, your options are limited, and you might want to use the rear brake as a descent control brake at nmoderate forces, and save the front for hard stops.