Caveat: I don't know that that is what they did here. But in other locations where they have pre-run the underground infrastructure for future expansions there are a couple of reasons why they would choose to do so. In some locations, their leasing agreement with the property owner has a staged structure where they get access to fewer stalls initially and then once the station meets some pre-defined usage rate trigger, they can add more stalls. The usage triggers are usually of the form X number of unique charging sessions per month for Y consecutive months. Additionally, it is better and more cost effective to have fewer stalls that are therefore full more often, so there's a cost benefit to not over-sizing the station. However, Tesla knows based on regional sales, existing ownership, and travel patterns that for some locations they will want to expand eventually and it is cheaper to do the undergound work all at once instead of in stages.