Distance and accessibility doesn't seem like much of a factor for AI's declaring war. In my pangea game, I can't figure out why Korea declared war. They were on the opposite side of the planets and the borders between us were closing up. They were actually one of the weaker civs too (1.2). I was able to get my army in and out, but I detected no invading armies. They surprisingly were late on walls with minimum requirement. This was quite before open borders were possible.
The notable second war was a 6v1 dogpile on the human. Only two could reach me(I needed Seafaring tech to reach China and an absurdly behind civ). Two more could theoretically attacked me though the ottomans who they were also at war with.
Also, dumb border gamerules got involved, but that's not an AI thing.
So, I guess pangeas on high difficulties means the AI declares dumb wars.