DR, well yes 521 was fully occupied in the normal business of landing, runway in sight, cleared to land etc. What the tape shows is that ops were proceeding normally; 521 cleared to land, 565 next in line cleared to continue approach. Then ATC sees something which leads to him instructing 521 to climb to 4000 feet. If that had happened 565 would have been given landing clearance but it didnt so 565 is obviously sent round again while 521 is sliding all the way down 12L.