An NPC joined a war and none of his allies are in the war. How was he called into the war? The war in question is a county conquest.
Thanks Naumfak for pointing it out that I was mistaken about single county conquest. That is indeed odd. However in case of a holy war or a prepared invasion the following stands true. If a reformed organized religion's (i.e. Islam & christianity) independent realm gets invaded by Pagans (e.g. Vikings), fellow Christian/Muslims will offer to join the war on defenders' side even if they are not allied to any belligerent party. For example if I as Petty King of Mercia get attacked by great Heathen army from Scandinavia, Other petty Kings, chiefs, counts, dukes, Kings can offer me assistance if they belong to the same religion as me.
Or if that NPC had a claim on that country, he could start an independent war, being hostile to all parties of the older conflict. For example, William the Conqueror & Harald Hardrada invading England at the same time.