Mainly it's bad choices.

I watched the entire event. Rubio was clearly uncomfortable. His leg started bouncing up and down when they started yelling: like when someone is having anxiety.

that happened to me once when i was watching a violent tv show. but i don't want to rule out other possible explanations in this case. maybe he was thinking about one of his favorite hillbilly jug-whistlin' tunes? or maybe he just forgot to pee before the meeting?