It is the "Vassal dislikes liege" events in rival_events.txt
In the upcoming patch we have reduced the amount of times these trigger.
I don't wish to seem ungrateful, but what would be REALLY sweet is if we could arrange that event to fire as a group.
Example:
#1) Three vassals approach you and say "I want a Kokijin" all at the same time.
#2) The player is allowed to pick ONE vassal
#3) The next time the player presses the "GRANT" button on the province, a window pops up and asks "Will you reward (Chosen Vassal) as you promised?"
#4) Player chooses "Yes" or "No." If "Yes" Chosen Vassal gets the Kokijin, and other vassals get the "I Got Snubbed!" penalty.
#5) If Player chooses "No," then Chosen Vassal gets the "You Broke Your Promise *cries crocodile tears*" penalty, other vassals get snubbed penalty, and player gets to choose who to give the province to as normal.
Have the event fire at the outset of a newly declared war, and maybe once every 15 months afterwards. Please don't fire that event when the AI successfully pulls off a massive plot against the player, though. That would be a major slap-in-the-face.