They did that purposefully!
You think the Queen's detail would not have known to the second when POTUS was arriving. They could have timed it perfectly and had her walk out as soon as his motorcade drove up. Or walked out after they actually arrived at the podium and made POTUS & FLOTUS wait.
But no, they instead chose to get Queenie out there waiting in the hot sun! oh, boo hoo! Look how bad Trump is!!
All very well choreographed!