Its very typical with poor screen quality. You will likely find that the glass is actually "popping out" of the plastic frame. You could try to glue it with B-7000 or similar adhesive, but it may happen again. Higher quality screens have hot pressed frames, so the frame is very securely attached to the glass and this doesn't happen.
The only other possible cause which comes to mind, which would be if the glass is definitely not coming out of the frame, would be a swollen battery