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I asked a similar question early in January, "why did my liege jump me?" and the answer I was given is that the AI will declare war on you if they have a claim, sooner or later, no matter how loyal you are or how good the relations are otherwise between you and your liege. I don't know if this is true but it is certainly consistent with what I saw. It's one of many details that I hope will be better in CK2.
What usually happens is that the AI declares war on his dukes/princes, and you help, and get some provinces to make your duchy prettier.
You just give out the provinces which border his own demense to some counts and that's it.