possiblities ?
1) Pads and disks were not "run in" correctly, when the new disks and pads were fitted, and the disks are now glazed
2) Inferior pads or inferior disks
3) Disk became (DTV) warped due to rapid heat and rapid cooling taking place in short time space (DVT = Disk thickness variance, similar to the way the tar gets bunched into a hump at traffic lights)
4) Suspension components worn