Lock Mart decided to upgrade their Vandenberg pads...
to support Atlas V launches because there's money to be made providing launch services for polar and sun synchronous orbits, such as big NRO payloads. The Atlas V is LM's big new booster. The Heavy version will replace the expensive and problematic Titan IV. It only makes sense that LM would provide the necessary launch facilities at Vandy as well as at the Cape.
If they can launch big stuff to a lunar polar orbit like they did with Clemintine (launched on a Titan II from Vandenberg), so much the better.