“Divrei” (“the words of”) denotes a harsh prophecy. The Radak cites a Midrash that three prophets – Jeremiah, Amos and Koheles (i.e., Shlomo) – start their books this way. They each spoke harshly to the people so the negative things in their prophecies started with them. Radak explains that this means that their books describe bad things that happened to them.