The most likely causes are uneven brake pad deposits on the rotors, or warped rotors. Uneven brake pad deposits can form as a result of track use without proper bedding of the rotors; they can also form if you brake really hard (like in a panic stop) or excessively (like on a mountain drive). Rotors can warp due to things like getting them really hot and then splashing water on them. Uneven deposits are often mistaken for warped rotors, as noted in
this white paper.
It is a lot more likely to be caused by the front rotors than the rear rotors. To cure the problem, in increasing order of cost, you can first try re-bedding the rotors and pads (the white paper has instructions), then getting the rotors turned, or, finally, replacing the rotors.