What is the Sunni justification for breaking the fast after sunset instead of after Maghrib?

I've noticed a lot of my Sunni friends being very particular about breaking their fast immediately after it is sunset. As far as i know, it is makruh to wait a long time to break the fast, and that it is not wajib to break it immediately. I'd like to know the Sunni justification for a couple of things: - Why break the fast after sunset instead of after Maghrib? - Why be so stringent about the time the fast is broken?