This is a great question that enticed me to watch that moment a fair few number of times. I’ll provide a lame, handwavy explanation and hope to be corrected.

The right foot was on the floor while the left was on the rug, which was in motion and and raised delta the thickness of the rug. In such a mode, the bot is inclined to prefer the stable right leg and lifts the left leg. (I wonder if it can balance on one leg indefinitely? If not, that makes this sequence even more impressive.) Putting the center of gravity over the stable leg increases the probability of the bot staying upright. As opposed to leaning or shifting weight with both legs remaining aground, lifting the unstable leg entirely from the moving surface maximizes the goal of staying upright by minimizing the force of friction. I also think the rug was jerked at just the right moment such that it maximized the visual impression.

This explanation avoids the need for the bot to predict visually the impact of the rug pull, since the rug pull was not instantaneous and the bot’s reaction came after the rug pull began.

This is a very impressive demo. I do think they nailed the goal of expressively and emotion.

It looks like the same corrective behavior as at the very beginning of the video when the robot is pushed sideways. It does a series of jumps to keep itself upright when its right leg moves too close to the center of gravity. Angular momentum of the body probably plays a role too.