1)
Why is this a new Dibur?
1.
Radak: Hashem ended the previous Dibur with "veha'Shefelah Yoshev." Now He returns to discuss the purpose of the first matters, which he should tell to all commoners, and what the first Nevi'im told them.
2.
Malbim: Now He will explain what were the first Nevi'im's words, and what caused the Churban.
