I’m quite sure these are not common otherwise we would have had a deluge of NO REASON posts about them.
I've seen plenty of posts.
What do you think triggers them?
Every poor outcome automatically triggers one of those. Repeated poor outcome(s) then trigger the real shocker where the freelancer loses the ability to apply for or accept new contracts, and I've seen quite a few of those too.
Upwork will be aware of the bounty of asshole clients and consider this in their algorithms.
Only if the client's feedback is excluded from the freelancer's JSS (client has a history of bad outcomes or is suspended).
I can’t help but think these would only be triggered by poor feedback from established clients with a history of giving positive feedback.
Nope, any client (unless excluded as above)