It is all being dug at once because the basements of all the towers are interconnected with utilities, parking and service ramps. Enwave Deep Water Cooling is also being built under the basement levels. The large retail component will also be a money maker when it is ready even if the towers above aren't complete, so they would want that income to offset some of the early construction costs for doing the whole basement at once. Also, the main truck access only has one ramp into the site (for loading, garbage to the completed buildings), so they need that access built to occupy any of the towers.
These are, to my knowledge, would be some of the reasons they are completing the basement all together.
