There is definitely a "chance to spawn" for the S ranks when a trigger happens. And I don't think that chance goes on a server-wide cooldown after it's been done once, because I've seen people catch Judgerays for the full duration of the spawn window of Thousand-Cast Theda, only to have someone's final catch at 09:03PM server time finally spawn it. Now, for Brontes, I can't say either for or against that just spamming the food is preventing something. Maybe it's the new acquisition of a food buff? Maybe it's a short cooldown per person? Sadly there's no way to test this and get verification.
As mentioned on our teamspeak before, I had gone through my weather data collection to look for Garlok's association with the weather and try figuring out of there's a pattern. I could not find one. I continually am looking for these associations for all the S ranks though as I am collecting every single weather change for every single zone and have been doing so for three weeks, coming up on finishing a fourth.
As for force pops, this seems unlikely. No way can it be possible for servers to not have seen Brontes for 6+ weeks, only to have him show up after the steak theory was published and they tried eating food for him. I saw a screenshot of 70+ people getting Seasoned Tracker all at once.
Try to prove it wrong. Actively tell people to not do the minion thing. Nandis longest recorded delay from his respawn of 48 hours is 8 hours 37 minutes. Meaning he spawned at 56 hours 37 minutes from the last time he was killed. If you can get that to go any further, minion theory might be proven. After 8 hours had passed beyond the respawn, said server started to spam minions in their impatience. Meanwhile Excalibur has non-stop been spamming minions previously and seen him last all the way to 8 hours 16 minutes. Make it go further. 9 hours. 10 hours.