Sometimes the aircraft that was supposed to leave a gate can't, because of delays or maintenance issues or whatever, so the plane that was inbound to that gate has to go somewhere else. This can set off a chain reaction as gates have to get swapped.

Last time I was in ATL our plane broke before boarding and it was just faster to move people from one gate to another, versus removing the old plane and bringing in a new one.

There are lots of reasons for gate changes. Delta owns the gates, so it's their call on who goes where.