Are you certain it was his speed varying, or was she trying to prevent him from passing? Not out of the question, given her prior controlling behavior.
In the video, beginning at 1:03, he pulls into the right lane. As he passes on the right, he looks out his window back at the driver. It appears that he then slows down to get next to her car, but slowed down a bit too much, so he sped up to catch up, just to flip her the bird.
Of course, none of us were there. I could be wrong.