Just checked your video!
Sounds like it was quite wind that day! So a little bit of movement in PH is possible for that she of quad.
Do you have a log file of this flight? Looking at the log and the video is most useful.
The 'rapid movement' are either due to vibrations (you can see it from the log) or too high PIDs (compare with the shaking of my quad when on 3S; PIDs were to hight for 3S use)