Like everything else, it's not the same for every woman. Some women have it on the day of O, some the day before O, maybe 2 days before, and others never see a dip at all.
The temps are correlated with hormone fluctuations. When the temps drop, estrogen is rising. Estrogen peaks right before O, hence the dip.
Edited for better wording.
Edited again because I was still missing some words. Believe it or not, I haven't started drinking yet!