What is the cause of the changes that the speaker mentions?

Attention, please. Due to expansion project, please note the following changes. All domestic departures will board in the main terminal building, at gates A and B. All domestic arrivals will deplane outside and passengers will be bussed into the main terminal. International flights will depart and arrive at the annex terminal, gates C and D. To get to the annex terminal, take the airport subway, which boards near gate A. Expansion work will last approximately 10 weeks. Thanks for your patience.