With your last reply, I would venture to say it's because she failed to leave enough space to slow down or move out of the way in the event she needed to brake/emergency brake, causing her to rear end the vehicle in front of her. This is usually given in rear enders, because in most of them someone has failed to give consideration for road conditions (traffic, road surface, speed, etc) and has then neglected to leave enough space to move out of the way, stop, or otherwise maneuver their vehicle.
Just my two cents.
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Any writings in this forum are my personal view and all opinions expressed should be taken as such; there is no implied or direct opinion representative of anything but my own thoughts on various subjects.