Why does this happen every so often for no reason. I'll turn device to watch a landscape video and the screen doesn't rotate. I'll go into settings and find it turned off.
Mine started to do the same thing after installing Nova Launcher. Don't know if NL is the culprit, but it does have a screen orientation settings (Nova Settings / Look & Feel / Screen Orientation) which doesn't seem to alter the outcome you mentioned. It's aggravating...