Because the diplomacy AI is broken. End of story.
Last night I started a new game, large pangaea with all 22 civs, emperor.
Met Germany on turn 3. Turn 4, he calls to tell me my military is pathetic (duh) He is now Hostile.
Few turns later, met Spain. Next turn, yep, she calls to say my economy is pathetic. Hostile.
Turn 15, an unknown civ has denounced an unknown civ. From this point on there are denouncements flying around every turn (but not against me, strangely).
Turn 31 Spain declares "war" though nothing happens at first. After a couple of turns a warrior+settler enter my territory. The warrior retreats after being bombarded, the settler continues until it gets captured. The "war" goes on for about 20 turns, at one point a swordsman and a couple of archers approach one of my cities and are easily killed.
Spain asks for a straight peace deal, no demands. I am not ready to expand yet (need to put some more happiness buildings up) so I agree. Next turn she calls to complain about me settling near her. The city in question was settled on the 2nd turn of the war. But despite this complaint, and the war, she is now Friendly.
The weird thing is on about turn 120, the denouncements are still flying around like crazy but no-one has denounced me yet. Not even Spain.