User blog comment:Miskos3/Fairy Tail 531: Pegasus vs. Black Dragon/@comment-7418318-20170415004809

I think Natsu activating Dragon Force is working like a beacon for Acnologia. This strong Dragon signature will make Acno lose interest in Wendy (and by extension all the people on Christina) for the time being and make him go directly to Natsu, Mavis and Zeref and that's how the crew and passengers of Christina will survive.