The cable guides and rollers are made of some kind of plastic. These plastic guides are aging, cracking and then breaking allowing the the cable to slip off the guides and the window falls to its lowest position. The cable is no longer tight and gets tangled up on the plastic take up wheel that the motor turns. 
Below you can see the stress fractures (black lines on the plastic) in the plastic on the window slider. Note that the cable is very tangled on the plastic roller. It is so tangled that the cable will not move. 
The piece that actually failed in this regulator. 