I don't think it was as innocent as that. I haven't really seen anyone saying what I think what happened, which is close to this:
Trump knows the cameras are pointed right at them. I think he realized how lost and helpless Carson looked and saw it as an opportunity to make a power play, not by "joining" him in his rebellion persay, and not because he simply wanted to go on last, but by instead acting like he's taking Carson under his wing, like he's protecting him from facing the situation alone. And plenty of people in this thread are sure that's what happened. And in a way it is what happened, but, as much as I dislike the guy, I don't doubt for an instant Trump knew the underlying messages that sends to everyone watching. One, he gets to look like a compassionate considerate person by being nice enough to not let him stand alone. Two, it pretty substantially diminishes Carson by creating the narrative he needs to be taken care of in the first place.
When you write it out, it seems like I'm attributing some mastermind grand plan on Trump's part but really it's a simple human experience I bet all of us face all the time: people choose to do nice things or help others all the time for entirely selfish reasons. It's just naive to think there aren't people out there who care more about the way in which they are perceived by society rather than the individual they're helping or showing kindness to or what have you. Trump saw an opportunity to show off for the cameras. It wouldn't take a genius to look at that situation and realize how people watching at home could possibly be feeling sorry for Carson and want to capitalize on that.
If the cameras weren't there, sure it could be just a nice gesture on Trump's part. But the cameras are everything. How many times have you been compelled to do something nice for someone else merely because you knew, for instance, your love interest was watching?
But don't get me wrong, Trump's still an idiot. Just an idiot on the look out to inject himself into the narrative at every turn.