User blog comment:ChocolateSquirrel/Questions About the Dragons/@comment-49.150.40.228-20140919010808/@comment-7418318-20140919113339

This last theory gave me an interesting thought.

If they are somehow linked with Acnologia too, that could explain things. The first time when they met Acnologia, the Dragon was just playing around. This time Acnologia has come to deal with E.N.D.

Maybe whatever emotion compells Acnologia - whether it's anger, fear or something else - somehow transmitted to the Dragon Slayer and caused this odd reaction.

That would certainly explain why it happened to all the present Dragon Slayers (particularly Sting and Rogue who's Dragons are supposedly dead) now, but didn't happen back on Tenrou.