Leave Sphere of Influence
It requires that the secondary power in question has a relation below 0 with its sphere master and that the sphere leader and Secondary power does not have a truce. The event removes the country from a sphere of influence as well as setting the relation to 'opposed'. It also gives the great power a 'add to sphere' casus belli, so the decision should not be taken without serious consideration.
Leaving a sphere of influence, without having another great power lined up, can leave the nation unprotected and have effects on the economy, since it is also leaving the common market of the sphere.