What makes tributaries ask to become client states?

[Fart_Gas]

When I played as the Antigonid Kingdom, tributaries such as Judea and Arados would ask to become client states. Why?

Also, why don't all of them do this? Some of my tributaries don't do this despite maxed out relations.

[Jean-Maurice Nya]

I think it's kind of random. You can increase the probability by befriending their leader and having a very high reputation.
But... it happened to me with nations I had very low rep.

[[__h.stickeye_]]

make sure their loyalty is high, and then there is a chance they will ask. One indicator this might happen is when they sometimes give you money for just being awesome.