Peeking at tuning I cannot for the life of me figure out what is causing this.
Eating alien pizza, eating alien candy, being affectionate with an alien, talking to an alien, marrying an alien, renewing vows with an alien, and specifically Searching for Truth with the telescope increase the alien abduction tracker. One of the tarot cards reduces it. Contacting aliens with the satellite dish guarantees a very high chances buff (permanent 10% chance per night, until an abduction happens), and being a scientist boosts that buff to 20%, and having space rocks and aliens on your lot bumps it up to 40%.
Otherwise it appears to be a random chance with a default 98% chance of not happening, but 2% chance of happening, that becomes a higher base chance the higher the hidden tracker (from the above listed interactions) gets, and then the chance further multiplies if they're a scientist or have space rocks or aliens in their inventory or on the lot.
Nothing I could find would make this happen more often. My guess is that something with the Tarot system busted it- since there's a Tarot card that affects abduction chances- but I can't find any relevant tuning.
(Note that using the telescope itself does not actually increase abduction chance, which is odd. I think it used to.)
Perhaps sims who had previously gotten the hidden abduction tracker to a certain level are now locked into that level for some reason.
If you are on PC, download Lumpinou's Sim Info Toolbox. Click a Sim who keeps getting abducted. Choose to View their data and find their Commodities. Check for the "commodity_AlienAbduction_tracker" and note its value. Once your Sim is abducted, check it again. Tell me what the values were. That would be my best guess, and it would explain why it isn't happening to every sim- it may be sims who already had a higher chance are stuck with that chance?