With Austria, this is not an event, it's actually a mission. I got it mid-game where there was a mission (top area, not bottom area) that said "Protect Our Neighbor Hungary" or something and it gives you a PU CB. I was pretty happy about it.
I can see that most people who always have missions pending might miss out on this.
You're confusing Hungary's events with Austria's missions.
There are two events for Hungary.
While Hungary has its starting Regency, there is a 90% chance that the AI will select to place "(Regent) János" as the ruler of the nation, with ""Ladislaus Postumus von Hapsburg" as the heir. Ladislaus will have very bad stats.
There is a 10% chance it will retain the Regency with "Ladislaus Postumus von Hapsburg" as the heir. Ladislaus will have average stats.
Once Ladislaus is of age, between 1450 and 1457, if there is no heir, there is an event with a MTTH of 1 month.
There is a 75% chance the AI will choose to kill the von Hapsburg, place Matyas Hunyadi on the throne, and give every nation with the Hapsburg dynasty a restoration of union CB.
There is a 25% chance the AI will choose to be placed in a personal union under a random nation which has a Hapsburg ruler.
If the AI selects the latter, that nation gets the PU and a small boost in prestige and legitimacy.
I've had that happen once as Austria.
When
Austria has more provinces than Hungary post-1500, they can get a mission that grants a restoration of union CB on Hungary. The same thing is true of Bohemia post-1500 and true of Burgundy at any time.