How does this work? Sometimes they fabricate claims, most of the time they don't. I look at their diplomacy screen and their diplomats are doing nothing?
More often then not they will fabricate on your rivals, but the most reliable thing I've encountered is to give them a piece of land in a peace deal. After that they will seemingly consistently fabricate claims on that nation, rival or not.
I'm playing as Sweden, my Orthodox vassal Finland (which doesn't actually own any Finnish culture provinces) has create half a dozen claims all along it's long border with Muscovy. I think what makes the difference is that they seem to make claims only on their overlord's rivals (Muscovy is my rival) and perhaps on provinces which are not cores, but of their primary culture
Yes it is true that it is the most reliable thing I've encountered. Emphasis me. Sometimes there is no claims, sometimes claims on rivals, sometimes claims on everyone, but most times I've noted if they are not already fabricating, they will start if I give them land in a peace deal.Na that's not true at all
I'm suspecting their rulers personality also plays a role. Militaristic might spam those claims all around, while diplo orientated ruler won't.
I would have said 'rivals only', but Provence, which I hold in personal union, keeps fabricating on France despite the fact that France de-rivaled me (and caused me to lose them as a rival) over a decade ago.
Remember that countries also get claims by event, so situations where you think they are fabricating, they might not be.
Playing Bohemia, I had Brandenburg as vassal for > 150 years but they did not fabricate single claim. Saxony expanded for a while before I vassalized them so I could feed them cores they made before. In general, I never saw a vassal to fabricate new claims, they stopped to actively expand once vassalized. I could only feed them cores from the past, or when awakening dead nations like Guyenne, Croatia, Styria.
I would have said 'rivals only', but Provence, which I hold in personal union, keeps fabricating on France despite the fact that France de-rivaled me (and caused me to lose them as a rival) over a decade ago.