Why is it bad to turn an engine backward?

I heard on some Nascar races where a car got spun around that it could damage the motor when the car spun around backwards.

This happened to Jimmie Johnson at Talladega going into turn 1 and Dale Jr got into him and he spun off to the inside of the track and spun and went backwards. No other cars were involved which was amazing by itself.

Then about 20 laps later his motor went out. Done.

There were other races where the same thing happened.
 
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