The screen assembly is made of a piece of glass to wich are attached a multitude of other layers (LCD, touch, polarizer, backlight, etc.), as we all a piece of metal and plastic called the frame of the screen. This frame is glued to the glass from one side using cold glue, and it sits into the iPhone frame from the other side. So in addition to the above possible causes of lifting, if the screen is of a very poor quality (so depending on where you purchased it from), the manufacturer could be using hot glue to attach the frame instead of cold glue. This will cause the glass to eventually detach from its frame. The frame remained clipped into the phone frame and screwed at the bottom, but the glass and the LCD screen attached to it lift up. This lifting can be exacerbated by a number of factors such as the frame of the iPhone being bent, or the battery being bloated and pushing up, extreme heat from the phone sitting on the dash of your car in the sun, etc.