LOL I love it. The Guardsman probably did that to scare the sh!te out of the tourists. The blokes on guard dismounted because their kit wasn't up to standard for public display mounted at Horse Guards Parade.
I remember years ago at Horse Guards Parade in Whitehall, a young thug kicked a Life Guards horse, the bloke got off his horse, rammed the yobo against a wall with his sword at his throat. It was front page on of one of the daily newspapers, the little yobo's eyes were open like organ stops as he was read the riot act. Brilliant.
There was another incident outside Buckingham Palace when Guardsmen on guard were situated outside the gates. A tourist was in a Guardsman's face began making fun of him. He slammed to attention with his hobnail boots, right on her foot. She was carried off screaming lol.
A mounted Life Guard said years ago, that he gets quite a few phone numbers of young ladies stuffed into the top of his boots whenever he is on mounted guard.
Those people deserved to be startled... in my opinon, they do.
The guards are there not for ornemental purposes, they are they to serve a function!
It is bad enough that they are gawked at by silly tourist, made fun off by idiots who think it is fun to do that. I remember one incident (I think I saw it in ouTube), a Guard smacking a young tourist for making fun of him during on of his marches. That young man deserved that. I would be sad if action was taken on the Guard!
He was probably charged for leaving his post.
Many years ago I was on public duties with rifle and bayonet along with a buddy of mine. Whenever one of us wanted to move to get the blood flowing, he or I would tap the butt of our rifle on the ground as a signal, count 1 2 3, come to attention, right and left turn march for a given number of paces about turn then move back to our original position on guard. However, after an hour or so I felt a buzzing in my ears, next thing I know I'm being carried off as I had fainted. My missus who saw me faint tried to run up with other people to help, apparently my buddy was standing over me with his rifle and bayonet in the "On Guard" position shouting "GET BACK, GET BACK. I don't think he was worried about me, he was worried in case my rifle and bayonet got nicked. My missus said afterwards that when I fainted I didn't crumple, I went over standing at attention lol.