A few of us in the office have found that if you lock your computer with Windows Key + L, when you come to unlock it, it might not recognise all your keystrokes and in fact seems to start Narrator. This is because Windows gets in a muddle and doesn't realise you've let go of the Windows Key about 18 hours ago.
Workarounds until/if Microsoft fix this are:
Change your password to not include a shortcut key using the Windows Key
Use Control + Alt + Delete to lock your computer
Use pin or picture passwords.
This seems to affect Windows 10 as well! Keeping using Control + Alt + Del to lock your computers!