Over the weekend I ended up getting a brand new set of Avid organic pads -- I wanted organic anyway and thought a new set of pads might do the trick. Did fine yesterday but today the brakes gradually tightened up on my ride to the point that I had to pull over and loosen the two adjusters. At that point, the wheel was basically locked up by the pads.
I will need to pull the pads again and make sure they are properly seated, but I was pretty sure they got seated well when I installed them. They clicked into place in a satisfying way (though I do find putting in BB7 pads to be a bit fiddly, as the British say -- I have trouble keeping the pad/spring sandwich together while getting the whole assembly into the caliper).
Really annoying and strange.