Why the event is random or is it?

I'm going to say "YES" they can happen only once because there is a counter in the Guide that says "0/24" (I think). There are a good amount though that are repeatable (these fall outside these specific ones though : ex cop chases, firetrucks responding
with lights and sirens, police choppers following a suspect).

There's also an achievement for taking people to the Cult's compound.

^ A blue bot pops up near you letting you know there is an event you can engage in.

In the couple of hours I've played I've had 2 events (security van hijacking, and a purse robbery)

Nevermind, situation resolved itself. I looked at my social club page & didn't see Antonio as completed, though it is sometimes hard to tell which is which from those tiny photos!

So I was heading back north in the blimp to see if she would reappear & decided to buzz the cult along the way. As I dived bombed them in the blimp lo & behold I was magically transported back to the cut scene in the altruist camp (complete with fiery pieces
of blimp raining down on the idiot cannibals) where Trevor finds the AK. This time I was a bit less gung-ho & methodically gunned them all down. I collected all the loot & parachuted down the mountain as my vehicle was not outside the gate any longer. Too
many mountain lions to walk out of there.

I then went back to save Antonio & she was there... all's well that ends well!

Hopefully this will help someone in the future.

I pulled up to a street corner and honked the horn. A group of gang members opened fire on me. Maybe my horn was wearing the wrong colors... ? I've ran into some strange stuff out there. Has anybody heard about the "strange disappearances in west blaine
county" on the radio? Aliens, or maybe Bigfoot? This game is awesome.