Here’s why I think he’s doing it. I can also see a lot of people switching lines so that they can receive directly from the pope rather than a “lesser” priest even though that doesn’t magically make the Eucharist any different. In several of the communion threads here on CAF there are people who will unabashedly tell you that they switch lines to receive from a priest rather than a extraordinary minister because in their minds (I’m not one of them, so I can’t justify it to you) it is somehow better or holier or something. How much more would this mindset apply when one of the people handing out communion is the pope? I can imagine enough people changing lines to seriously mess up the distribution process. The pope’s line would be twice as long as the other one, people would either have to cross in front of each other or take the long way around to get back to their seats, and, when they get their seats, they’d have to climb over their neighbors who, having taken the shorter line, have been seated and praying for several minutes already. It would be a mess.