I've had my brake drums off recently for pre mot inspection, I noticed the handbrake balance bar was pulling to one side, with the drums off, I found a piece of metal from the auto adjuster floating around inside, luckily no further damage and I have ordered and recieved adjusters shoes and wheel cylinders ready to fit over the next few days (weather permitting) So wonder if something similar has happened to your brakes (and its not just a stone stuck in the tyre!)