The Kindle is unique in screens in that it utilizes an E-ink display. In this display, the pigments on the display are made of small particles that are sensitive to charge. From what I understand, pages are encoded as a matrix of charges, which cause the pigments to turn black, white or in between. Broken displays will encode some of the correct data, but will encode some wrong data making a sort of collage of different parts and pieces. Seems also like it encodes data from other pages and functional screens.
The effect produces a really sensitive image of the death of these devices and their physical (chemical?) nature.