If things are going to happen a certain way--no matter what--that's not a game: that's a book or a movie.
Well, I can see the sense in making a distinction between "setting events" that are going to happen regardless and "scenario events" that whether they happen depends on what the players do. And even some few that are borderline between the two.
As examples, by the local method of measuring dates, the volcano Mt. Morris will erupt on August 16th, 1458. The game begins 150 miles southeast of there in a small town on January 1st, 1458, and the most likely scenario will take the party to a spot 50 miles northeast of Mt. Morris.
The eruption of Mt. Morris is a "setting event". Yes, it's barely possible that powerful enough magic will prevent the eruption, but only if the players find out it's going to and have such magic available. I think that can be safely counted as "not gonna happen".
The attack by fire giants in the ruins of the town 50 miles NE of Mt. Morris is a "scenario event". If they don't go to the town, it doesn't happen. If they see the scenario unfolding better than the GM expected and enlist the help of some mortal enemy of fire giants (who go and attack them themselves), well
technically it still happens, but the players may all be many miles away when it does.
The magical plague that strikes the major trade city is borderline. Unless something happens to prevent it, the evil cleric will drop the plague item into an exotic dancer's tip jar on the night of March 29th, 1458. She's an elf girl, she's unlikely to be affected, but as she walks through the crowded market on her way to get the item identified and appraised, everyone around her will be.
If the players have any dealings with the evil cleric before March 29th, the plague may not happen. If they have any dealings with the exotic dancer, it may not happen
on March 29th.So if they meet the cleric on the road on the 28th and kill him, and then succumb to the plague when rifling through his belongings, "The Great Plague" becomes a couple of dozen people dead a day away from the walls, rather than a couple hundred thousand within the walls. If the elf girl went off with the party and only humans are working the night of the 29th, the evil cleric has no "Typhoid Mary" to walk through a crowded market with no symptoms, yet causing disease in others nearby, so the evil cleric has to choose a new target to get the results he wants. The plague will likely still happen, but not on the night of the 29th.