"15 states" did not start petitions. Individuals in 15 states started online petitions that anyone in any state can sign and that are of absolutely no force under the law. All reaching the magic threshold of 25,000 signatures gets is a formal response from the White House, not placement on any ballot. If the threshold isn't reached, the petition is removed. Secession, even if it could actually get through some sort of state process, would still have to overcome a Supreme Court challenge, assuming it ever got that far.