It was a political rally. Those are always done with flags. It was a political rally, there were a bunch of cameras running. Many of which were taking dozens if pictures every second; there wasn’t even anything to release there, the media was the ones taking the video. Of course the most striking image would be the one to catch on, and from watching a video of it, that seems like the obvious moment to take.
It was some fast thinking and good political instinct (although bad survival instinct) to make a photo perfect pose while getting escorted out by secret service.