Could be an install issue or it could be a defective belt. This is why we provide an extra belt free of charge. We know things can happen. Please be sure the pulley tight and secure.
Speaking of belts breaking... Just last week my 135 belt just blew up after 30k miles and bmw installed a new belt which failed within 3 miles. Its been a week since the 2nd belt no issues. I would bet it was an install error as it is very uncommon for belts to completely shred in such a short amount of time.