I think it's because he's in a quiet room. You have a tendency to speak lower and slower and relaxed when it's a bit more of an intimate setting than when surrounded by people and noises, and in his case-having to fill various mics, etc-which is when your voice will go a bit higher in register in an attempt to be heard over other people or minor background noise, in general. Not really yelling but projecting a bit more from your diaphragm than you would need to do in a quiet setting like this. It's instinctual and most of the time you probably dont even realize you do it. Most of what we see of him involves him having to project quite a bit-acting, mass scale interviews on chat shows, panel appearances, etc.