Why does my Xperia Z Ultra lock randomly?

[AC Question]

Xperia z ultra locks randomly

I'm experiencing this since yesterday.. My mobile gets locked randomly.. If I move an app to SD card it gets locked, if I download an app from play store it gets locked.. It's annoying to unlock every time..
I even ran the mobile in safe mode but the problem exists...
Did anyone came across this problem?
Please reply..
Thanks in advance....