One slight movement can cause your iPhone's screen to rotate. This can be really frustrating as you immediately lose the point where you were in the app. The fix might be easier than you expect.
If you lock the screen orientation, it will not change when you rotate iPhone.
Open Control Center, then tap the following icon > .
If the screen orientation is locked, the icon > appears in the status bar (on supported models).
No rocket science here, hope it helps :)