User blog comment:FairyRaven/Fairy Tail Chapter 327: My Part Too, Review and Commentary/@comment-203.87.139.100-20130412131954/@comment-7418318-20130412134207

Good point... how did they know about the gate so fast and why are so many coming through.

Could it be that the dragons that come through are actually fleeing from something that happens (happened?) on the other end of the gate? Could they be fleeing fron Acnalogia, Zeref or the past Dragon Slayers? That would certainly explain why they just rush through, rather than trying to take and hold the ground around the gate.