I think its Avast prob.
I keep Windows Defender disabled i.e when I try to open WD through start menu, I get a window that WD is disabled, click here to turn it ON.
I have checked with no AV or security system installed & restarting the system 2-3 times, WD was never enabled by Windows, instead I got notification from Action Center that WD was disabled.
2 days back I installed Avast Internet Security & system restart & WD was enabled.