User blog comment:IamJakuhoRaikoben/Chapter 294: Battle of the Dragon Slayers, Review./@comment-5343732-20120806084516/@comment-211.31.12.114-20120806111356

Pretty sure they can activate Dragon Force because of their lacrima which may have come from their dragon parent. They probably caught their dragon trying to run away and they decided to kill it and take the lacrima to become 3rd gen DS