If you look at this longer clip from CSPAN, and wait until the camera zooms out towards the end, you can see that he was walking across the stage to say something to the other woman who was sharing the stage with them. I’m not sure who she is - possibly another speaker, or the person who was serving as MC and introducing the Bidens. Either way, it’s clear that he has a quick word with her and then walks back to his spot.
If you ever wonder why you sometimes get mistaken for a troll or a hack, bookmark your post as a reminder.