Maybe a loose connection or a loose/weak backcover magnet? The first being probable and the latter being unlikely.

There's a also a mechanism which unlocks the screen when you slide out the keyboard, but that takes you straight to the Hildon desktop and skips the lockscreen, so that's definitely not cause of your problem.

If it were a software problem, then we would've had more reports of this happening, but maybe yours is an isolated case. Got any modifications or packages which mess with the accelerometer?

I also observed that the notification led shines white when the screen turns off. It doesn't indicate a faulty/loose screen flex ribbon, simply because it doesn't have the hardware to do that.
It's the same led pattern which shows when you press and hold the power button, though, so it might actually be a fault within the power button itself?

Can you install the led pattern editor, modify the poweroff patern (e.g. change the colour from white to red) and then check whether that changes the colour of your led when it happens again?