How not...

...to long line.

The handler allows the outside line to drop too low. It is not just useless in that position, but dangerous. The horse turns (because he thinks that's what is wanted) and his outside leg swings into the ankle-height rope. Then, of course, he panics.

The lines were correctly attached, although I would have used the next ring up, but the outside rein was just too loose and too low.