New York state election law generally lets state party organizations file paperwork that permits members of other parties to enter that party’s primary. But the law explicitly says that for the three citywide offices in New York city, including Mayor, the paperwork must be filed by officers of the New York city party organization.
According to this story, the Working Families Party New York city organizations did not file the paperwork for Bill de Blasio (the incumbent, and a registered Democrat) to enter the Working Families Party primary. Instead, the article says, the state WFP officers filed the paperwork. The article says the Reform Party will challenge the WFP nominations for Mayor and the other two citywide offices.