At noon, the sun is at the highest position in the sky above the observer. In order to reach that position the sun keeps going higher and after noon it starts going down. Therefore, the sun looks as if it stops for awhile at this azimuth.

Strictly speaking this 12.00 o'clock position is only happening in the middle of each time zone. If you are away from the middle position, the azimuth (highest sun) is not happening at 12 exactly, but a few minutes before (to the west) or a few minutes after (to the east) 12 noon.