Ari's pretty spot on. The rumble caused by the hookah will bounce the coals or move them a little. Because of the shape of the bowl, the foil going across will loosen a bit, especially when you poke holes. While Ari is right that "gravity" will keep the coals from moving too much, the inward slope caused by the foil loosening will gradually move the coals to the center.
A tip to help out, in conjunction with a base protector, floor mat, defuser, etc., is how PatelBadboy tapes up his foil. Watch this video.