Look in your server logs (probably /var/log/nginx/access.log) and see if there are lots of POST requests to /xmlrpc. If so, try googling for solutions. I'm not a php or wordpress guy so just learning about this myself... a client linode running wordpress was getting hammered yesterday and CPU usage was really high all day.
THIS! I just had this happen the other week on a LAMP server that I had at another provider...CPU Usage was going nuts and was even higher then yours OP, turns out it was XMLRPC getting pounded, there's Wordpress plugins to disable it or you can disable access to it via nginx configuration/htaccess for apache.