"Driving off a cliff" can be a literal phrase, referring to the act of a car or vehicle falling down a steep, vertical face of land, or a metaphorical one, signifying a sudden, drastic, and negative decline or loss. Link
It can describe a situation where someone or something is rapidly and significantly failing or declining. Link
The phrase is best described as an expression for a decline in whatever the situation might be because literally, no one would drive off a cliff except it is an accident.
So, when a thriving business or a chart reading high suddenly begins to fall, it is safe to say the business was driven off the cliff. And once it is off the cliff, it is only a matter of time before it hits the ground and damage occurs. Driving off the cliff can be a stunt, but it is a dangerous one.
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