Because one tire probably got some air time, while the other was still slowing the car down. There's a real rough section of road near me, and whenever I go over it and have to hit the brakes, mine do the same thing. If your pads still make a stink, take your rims off, and get some Brakleen and give everything a spray. They will be a little spongy if you jump right in after spraying, but they will dry and be fine.