Your phone reads gravity, not your intention
Inside a phone, motion sensors track how the device is moving and how it is angled. One key clue is gravity: its pull gives the phone a reference for which way is down. The operating system uses that information to decide whether the display should stay portrait or switch to landscape.
That works well when you hold the phone upright or turn it deliberately. But the sensor can report only the phone’s orientation—not whether you meant to rotate the screen. If you roll onto your side, shift the phone across the pillow, or tilt it while adjusting your grip, the device may cross the software’s threshold for landscape even though you are still reading in bed.
A little motion can tip the decision
The phone does not normally switch modes for every tiny wobble. Its software smooths sensor readings and uses boundaries between orientations to avoid constant flipping. But near one of those boundaries, small movements can change which orientation seems most likely. A movement that feels minor to your hand may be enough to make the phone settle on the other layout.
The problem is one of interpretation: the same physical turn can mean “show this in landscape” to the phone and “keep the page where it is” to the person holding it. The device has no direct way to know the difference.
Rotation lock removes the guesswork
When you lock orientation, you tell the software to stop using those sensor readings to change the display automatically. The phone can still detect movement, but it keeps the screen in the chosen orientation until you unlock it or select another mode. That is why rotation lock is especially useful for reading while lying down: it prevents a shift in your posture from becoming a change in the page.
