Brake Grinding Noise problems of the 2004 BMW 325

While driving at highway speeds, the car began to pull/swerve. A metal on metal grinding noise also began. I was able to get off the highway and have the car towed. A mechanic found that the rear passenger side parking brake had engaged and disintegrated into several pieces of metal. All other wheels were unaffected. The parking brake lever was down, and had not been used in several months.