We can over-analyse such things. But that rotation is usually typical of the early stages of acceleration as the athletes tries to make room to advance the pelvis, while integrating the need for power and rate within the stride sequence.
Might be that Powell forces his pelvis too far forward in the process of trying to keep his knees up during the latter stages of the race, causing him to rock back somewhat and shorten his stride. So he may be trying to push for Triple Extension, rather than that choppy, vertical action he falls into when closing on the finish.
Or he just might have been thrown off his rhythm and lost balance a little bit. It’s all in the heat of battle. Hard to be critical, although that’s the coach’s role I guess.