Sometimes (randomly as far as I can tell), my Mac mini refuses to go to sleep. I press the Sleep item in the Apple-logo menu on the menubar, display turns off, after several seconds computer goes silent, the power indicator starts flashing. It flashes for a few seconds (like 2 or 3 seconds), and then computers wakes back up.
pmset -g assertions output is the same right before pressing sleep and after it wakes up:
2015-05-05 09:31:13 +0300 Assertion status system-wide: BackgroundTask 0 ApplePushServiceTask 0 UserIsActive 0 PreventUserIdleDisplaySleep 0 PreventSystemSleep 0 ExternalMedia 0 PreventUserIdleSystemSleep 0 NetworkClientActive 0 Kernel Assertions: 0x4=USB id=501 level=255 0x4=USB mod=04/05/2015 13:13 description=EHC1 owner=AppleUSBEHCI
The timestamp is different, the rest is identical.
As far as I can tell, it shows nothing of interest, though I'm not sure about the last line. Any ideas what might be the cause of this behaviour and how to fix this? So far the only thing that helps me is rebooting, but the problem keeps coming up and around in a day or two and is really annoying.
I'm running 10.10.3 and Mid 2011 Mac mini.