It looks like when a little shock occours the phone acts like power button is pressed , but it's weird because another shock just shuts down the display , wich is impossible with any button without unlocking it first .

Here's a video , I dropped it few times on my bed to show exactly how it happens .

https://www.youtube.com/watch?v=yW5365VStJI

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?

nice transition anyways did you try to flash it first?

Maybe the sudden jerking movement is triggering the lock switch, or slider GPIO switch.

That white LED breathing when the screen is off is actually a feature.
I have it on all the time to remind me whether my device is on or not.

It is a feature indeed, but it is not turned on by default and OP has not mentioned activating it, so I wildly assume it is currently set to off.

Regarding the slider:
Same thing with the lock switch, so that's not it.

Let's just wait for OP to log back in and post results of the pattern editor test (and hopefully some more information on installed software too! *nudge nudge wink wink*).

Well , that led was blinking because I enabled it in settings . I don't have any app that interacts with lock/unlock or power button .

This problem occours only when the device is locked . It looks like the flex cable is broken or that plug came out from the socket . I will get some philips screwdriver and disassemble it to clean that flex and connectors . I'll come up with results .

What looked like a hardware problem turned out to be some feature or app I don't know about it . After I disassembled it and cleanned those flex connectors and removed dust , I've noticed the problem occours when proximity sensor was covered and released , it shows lockscreen . Now I restored to factory settings and is fixed .