There seems to be almost no correlation between actual relations and being denounced, haha. In one game as Japan, the US quickly became a runaway civ and attacked 3-4 other powers on their side of the map. They maintained friendly relations with me (we didn't come close to bordering one another at the time) and offered me repeated DOCs. Then, out of the blue, 2 turns after a DOC, he denounces me - 8 turns later is friends again, and another 25+ turns later denounces me again. I had no idea what made him hate me so, as the only diplo hits I had with him were regarding the fact that I had made DOCs with his enemies.