User blog comment:Stone Roger/Natsu and Zeref have somekind of relation?/@comment-24819648-20140415120030/@comment-4829046-20140424133939

So many theories for that but my personal pick is that he simply came there after sensing Zeref's "awakening"(lol) and the FT mages just happened to be there. He wasn't even being serious. Or Zeref subconsciously told him to come and kill everything on the island. Still all speculation as of yet.