Some of them are chain triggers, which are events like a heist or a war between two mafia families. These will appear randomly, just like the other cards.
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Lol, I got some comments on my game pointing at the exact same thing. I, likewise, had events that would get added to my random pool depending on previous events / decisions you had made. I wonder if maybe we both ought to up the likelyhood of encountering those "follow-up" events significantly so you're more likely to see the full story for those layered events.