User blog comment:Miskos3/Fairy Tail 520: Dragon or Demon, Review/@comment-4933386-20170205092919/@comment-1359322-20170205120208

From the stream translation of what Igneel said. The seed growing is a result of Natsu's distress over what he is. I suppose Zeref revealing Natsu being an Etherious cause Natsu to doubt what he is and the Demon Seed surfaced itself as a result.