Anyone who has studied ice ages would know that during the last two-and-a-half million years of Earth's history, there have been periods where there was enough ice to suck up a lot of water. As a result, sea levels fell by an average of 100 meters. Among the most famous examples of this sort of drop is Beringia, the landmass that connected North America to Asia.
But in the event that a worldbuilder is creating a timeline that parallels the Maastrichtian, the latest Cretaceous period, in which sea levels dropped at a time when the influence of ice was minimal--alpine, at the very most--what else could reduce global sea levels if not ice?
And please, keep it natural--no human or alien intervention.