As shown in the video, the tourist with the yellow dress and sunglasses appears to be standing on the wrong side of a rope which designates an off-limit-area to visitors. The woman was apparently joyfully posing for a picture in the way of the oncoming Guard of the Queen.
The marching soldier approached the woman from behind and shoved her with his right hand while steadily holding his rifle in the other hand. In total shock, she fell forward as she screamed and fearfully grabbed onto a random man who was standing among the crowd on the safe side of the rope.
Then, the soldier simply kept on marching forward at the royal palace where the wedding of Sussex’s Duke and Duchess took place last May. But that’s only one example of what could happen if someone interrupts a Queen’s guard.