User blog comment:IamJakuhoRaikoben/Chapter 431 & 432: My sword is... & Brair in Love, Review./@comment-24546844-20150429200634/@comment-9595671-20150430001831

I think its the Dragon Force itself that causes the transformation as like Zero mentioned is when a Slayer is truly equal to a Dragon. As Laxus and Cobra's DS magic come from Lacrima they most likely don't have the Dragon Force because the Lacrima is artificial and lacks the knowledge of the Dragon Force which would make sense as I don't think the Dragons themselves know how to use it.

Also Laxus and Cobra only use it rarely more so as they have more preferred magic and their DS magic is only when they're in a last resort/losing battle type situation.