Look under Schedule. You may find there is a setting telling the iMac to turn on
My 4 year old iMac, running Yosemite, just started turning itself on late at night. The startup chime wakes me up. The past 6-8 months I've been allowing it to go to sleep at night (when finished using it for the day) as opposed to shutting it down for the night using the drop-down menu from the toolbar. Before that, I always turned it off at night via the drop-down.