Could be many things really. First step is to update to the latest firmware, which is here. (If you look at the drivers page or tell the printer to check for the latest version you will get one vastly older than what is at my link)
If that doesn't fix it, unplug the network cable and wait, if it does wake up, it is waking itself up, so the Energy Saver is the issue or it is trying and failing repeatedly to report to Xerox its meter reads and supplies.
So if it doesn't wake up when unplugged/disconnected from the network, there is something on the LAN that is pinging it for some reason. Some security scans will wake printers, some will actually end up printing data when they do)
If it does wake up when unplugged, check that Remote Services works (assuming time/date and DNS are all correct, it should), or disable it if desired, so it will stop trying
Make sure it isn't set to delete stuff every 1.5 hours (Because it will wake to check) via EWS > Jobs > Job Policies
And see if disabling SNMP on the device makes any difference
The WorkCenter 6515 has the annoting habit of constantly waking up and running through it wake routine every 1 1/2 hours or so. Goes into standby, sleep, then does it all over again 1 1/2 hours or so later... 24/7. Have found no answers on how to stop it, or even anyone else with the same problem... Its across the hall in the den, but it is noisy enough to disturb from across the hall. Right now it just gets shut down after lights out, then turned back on in the morning or when you go to print something.