Covet lands problem?

[Ajuga]

In one of my previous games I was playing as Askia.
I was located in the upper right corner with city states surrounding me. China east of me beyond the city states and Spain south of me and the city states. (South-west was sea.)
Now below Spain we had Japan with west of him Darius and south-west of Darius we had Denmark, Siam and Napoleon but don't mind those three.

Spain and China went to war with me once and Spain, Japan and Darius were fighting with each other.
Somewhere around 1500 AD Japan was beaten by Darius. Before his complete death he managed to sneak a city beyond my city states just next to my border. Because we were friendly I didn't care too much.
After a few turns Japan went hostile against me with the 'covet my lands' modifier. (Also my pointy stick was the second highest in the game with only France being bigger.)

Why is it that he coveted my lands instead of coveting his own lands that were taken away by Darius? Why start a fight with me?
Even though I technically made sure he survived as I could have easily not grand him open borders which I rarely do.

 

[joncnunn]

Well; your mistake was allowing him to found the new city on your borders.

Land envy is largely based on distance to capital.

Moment his city on his border became his capital -> major land envy with you.

 

[Heerlo]

Once he got closer and saw how good your land was, he thought, 'I want that land!'

Just joking.