they may not be as arbitrary as you think. You may be pushing up against their boarders, becoming friends of their enemies or trespassing in City-States that they are allies with. It's a lot less arbitrary than a game like Fallen Enchantress.
It says when I'm pushing up on their borders. What I get irritated at is when it's just "You're a d**k, I'm telling the world that you're a d**k" But that's it, no reason why.
I've learned that there's NO reason to allow open borders unless I initiate it. 75% of the time, it's a ploy to get close to my cities and attack. The trading also seems random...
Them: I want 500 Gold, here's 3 candy wrappers and a used tooth brush
Me: Uhhh, how about 500 gold for a horse
Them: "That's not even close to fair! 'how DARE you, this is an 81 Honda!!'"
Them: Can we have open borders, pretty please?
Me: OK, but since my land/army is 72x the size of yours, how about you give me 5 gold?
Them: You're crazy if you thnk that's on the same planet as fair
Me: Hi country of any relative size of my own, wanna do open borders? What'll it take?
Them: 25 horses, 500 gold, 20 silver, 10 uranium, 1st born child, and 2 nights with your wife
Me: How about all that, but only 495 gold instead of 500?
Them: I declare war on you!!!!