Time:  Time is their enemy. Over time the image and sound quality degrades.

Stretching:  The normal use of watching the video, fast forwarding, and rewinding the tape causes this.

Mold:  The environment of an enclosed cassette attracts a white powdery mold. This degrades the video and audio quality.

Warping:  Temperatures fluctuations of too hot and too cold can cause the tape to warp. This ruins the audio even to the point of hearing static noise. It also makes it more difficult for the player to play the tape at a normal and consistent speed which ruins the video and audio.

The magnetic charge:  The particles in the tape that create the video and audio lose their charge or the charge becomes less. This causes a color shift of the video picture.

Humidity:  When there is too much humidity in the environment it is absorbed into the tape. When it does the emulsion sticks to the other tape that it is against and peals off. This is called Sticky Shed Syndrome.

Mechanical issues:  The tape getting jammed in the machine and then pulled out, cassettes being dropped, the tape breaking making it necessary to splice, and many other factors.