If you look under an angle on the display (i.e. from the buttons upward to the top), then you can see a little space between the surface of the display and the aluminum frame surrounding the display. This is where crumbs (i.e. by moving dust underneath the aluminum while cleaning) can get in. Those cause the touch layer to "think" that the display gets pressed. So it's not reacting anymore because it's going first come first serve.
Hope this helps understanding. If not, I can make a little drawing to explain a little better.
Let me know.
