When your zetaruler is born, like Chassit16 says, a d-100 is rolled and subtracted from 100. The adjusted roll is checked against all your unions' inheritance chances on the ruler's death. If the inheritance chance for a union is higher than the roll, you inherit that union. Since the same roll is used for all unions, if you inherit your lowest-chance union, you will also inherit all unions with higher chances.

The only way to savescum this is to increase chance of inheritance over whatever your roll is.

Try picking up more diplo rep from an advisor, papal interaction if you can afford it, and policies if they're available. Enforcing culture on unions with different primary cultures (or culture groups, can't remember) also will give a small bonus to inheritance chance but costs diplo points.