"Past Grievances" always with Vassals

[Ahiga]

I'm not sure why, but no matter who I vassalize, even if I didn't encounter them prior to their destruction, they always have very hostile (around -100) "Past grievances" with me, the Chosokabe. I'm not sure why they have a grievance with me, since I vassalized them from being ruled by someone else?

[walkerk19]

Yes, I had the same problem also.

[Becephalus]

I think that is there to make sure you stay nice to them. Otherwise it would be too easy to hold them forever.