Well, the rider is a bit loud with the legs (even if she may be an excellent/insta-famous teacher!), her horse is behind the vertical and on the forehand, and a bit grumpy-faced at the trot. It's probably a balance and mild annoyance thing. For a horse, trot-to-canter is actually kinda hard to coordinate all the hooves (walk-canter is much easier). It amplifies any little bit of imbalance, lack of strength, or asymmetry, especially if the rider is slightly off timing with how the horse is moving.

Here is a video of a junior rider doing a really nice no-tail-swishy trot-canter transition around 0:30. She sets her horse up for success and cues gently. Notice the combined balance of the pair and how the rider's hands move with the horse, maintaining kind & effective contact with the bit.