My BB7s were doing that about a year ago. Pads weren't that old, -just one winter's worth of use. Replacing them solved the problem.
I must say that as a relatively new user of disc brakes, they aren't as great as you all had lead me to believe.
They stop well enough but I've had the above problem along with horrible squealing when wet. The rotors have needed some truing and initial adjustment (on install) was a pain to get right. Why wasn't I informed?