The changing of the guard had been happening every 20 minutes so it wasn't due to standing still for long periods. He had stepped off the podium a few moments before collapsing, perhaps feeling unwell but then returned to his position.
He blacked out and fainted forwards, smashing his face off the stone floor. You can imagine the gasps from the crowd but the other guards don't come to his aid as they have to remain still and in their place. The police ran over to help and the live stream was cut off for several minutes.
