Possible reasons:
http://stackoverflow.com/questions/28823254/too-many-nginx-waiting-connections
https://forum.nginx.org/read.php?2,49182,49196#msg-49196
https://forum.nginx.org/read.php?2,261632,261664#msg-261664
http://stackoverflow.com/questions/28823254/too-many-nginx-waiting-connections
https://forum.nginx.org/read.php?2,49182,49196#msg-49196
https://forum.nginx.org/read.php?2,261632,261664#msg-261664