Weight has little to do with it. An automobile's steering geometry is all wrong to be pulled backwards with the front wheels on the ground. The caster angle and amount of steering 'trail' that makes the steering natually want to center while rolling forward causes it to become intrinsically unstable moving in reverse.
Think about how responsive your steering is when attempting to back up at high speed. Now imagine doing it at 70mph.
All about caster from
Wikipedia
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Originally Posted by Wikipedia
The interaction between caster angle and trail is complex, but roughly speaking they both aid steering, caster tends to add damping, while trail adds 'feel', and returnability.
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