he was driving a bit close is all, speeds weren't all that high. And he could have been having an animated discussion on bluetooth phone when stopped, dunno. Lucky the instructor didn't decide to test for emergency stop.
I heard of a driving test assessor explaining that during the next few minutes, when he taps the dashboard sharply with his clipboard he would like the driver to do an emergency stop. The assessor subsequently dropped his pen, and when reaching down to get it , tapped the dashboard with clipboard. He got a bloody nose, but still passed the driver.