I thought it was because of the rain and they were prepared to hold an umbrella over his head as he spoke, and to make sure his surroundings were as dry as possible. I didn’t think much of it. They retreated when it appeared it was merely a drizzle and President Trump was doing fine by himself. He’s not a little snowflake who will melt with the first raindrop.
I thought it was to be able to evacuate the elderly former presidents and First Ladies first in the event of a downpour.