The ninjas have been hyped up since the Zou arc with Luffy and others fanboying over them and at Wano you can find several ninjas lurking on the background of panels. Oda has been showing his love for them and I imagine he has something big planned on the story. Shinobu says that when Orochi took over Hyogoro, leader of the Yakuza, refused to bow to the Kurozumi family and was executed while Fukurokuju, leader of the Oniwabanshu, chose to serve. But what if the ninja betrayed Oden even before that? Ninjas always act throw the shadows and are loyal only to their goals, not caring for the means to reach them. A ninja's purpose is to complete his mission, so what could the Oniwabanshu's mission be? To protect the Poneglyph of Wano. Oden's dream of opening the borders of Wano was detrimental for the ninja's objective of keeping the Poneglyph secret and safe. Having foreigners at Wano would mean it's secrets would eventually be discovered making the country a target for the World Government. The ninja feared Wano would have the same fate as Jaya. The Shandians were trusted with the protection of the Poneglyph in Shandora, but the city eventually fell in order to protect it. Oden was loved by the people but he died executed as a criminal. The Oniwabanshu joined forces with Orochi from the very start, helping to frame Oden. This betrayal was the key factor for the events of 20 years ago leading Orochi to power so he could spread his propaganda against opening the borders. In the current timeline, there is suspicion of a traitor leaking the alliance's plan and that can also be attributed to the Oniwabanshu since as former servants of Oden they would also recognize the coded message.