The problem is that the 60-to-90-foot-long friction piles underpinning the building were driven into sandy soils rather than bedrock at 200 feet down. While no concrete explanation has been given for continued sinkage, developer Millennium Partners has blamed construction of the neighboring Salesforce Tower for pumping out too much groundwater and causing the soil to settle.

Source: San Francisco threatens to block access to Millennium Tower – Archpaper.com