because they lost a war.
Every time I get someone to capitulate it means that they lost all their cities except for 1-2 to me, I then often liberate most of those that are in their core/historic areas (basically back to the status they had before the war) but that doesn't change their status from collapsing/unstable to anything else.
Of course there are civs like the Inca that are just designed to be unstable, but the Aztecs, Moors and other civs are also always collapsing by the time they are willing to capitulate.
I gather they have a high military stability malus plus some problems in other columns due to the war they lost to me.
In my last games I had always stopped to try and vasallize someone because that would just lower my own stability (I can deal with the malus for overextension but I can't counter the foreign stability for having an unstable vasall) and instead just destroyed the civ, because that was better for my stability. However that just feels rather weird to me and also is often quite annoying.
That's why I think this might help counter this temporary stability malus these civs have after losing a war.