While Hailes technique is nearly all but perfect, he still has small faults that coincide with his achilles and foot injuries.
As you can see, the first thing is his overstriding and landing with his foot past the midline of his body. This leads to the body having to catch back up with the foot when the foot lands. While this won't cause much problems in a few strides. It becomes a problem when a distance runner repeats this over thousands of strides per day.
In each stride he pronates, which is more noticable in slow motion and even better in the barefoot clip. His big toe is unstable which leads to his pronation, which then leads to his knee SLIGHTLY collapsing medially.
We are talking minimal problems here, but overtime they become big problems.