This is the result of "years" of activity--usually.

Chewing doe NOT cause damage to teeth.  Your tooth to tooth contact while
chewing food is very light.  Studies show people can clench with over 200
PSI of force while awake (some studies show much more force is possible).
Other studies show that muscle activity intensity during sleep can increase
14 times, during sleep, for those patient who clench at night.  Add to the
mix that most night-time clenchers will grind isometrically while sleeping.
If you are conservative and allow only 250 PSI of force during sleep, you
are doing the same thing as if you were balancing a piano on each tooth and
twisting it with its full weight on the tooth.  Do this for 5-8 hours a
night for a few years, and guess what happens?