Musk has always been this way but was never really in the public eye so people only knew him as the mega rich dude that made futuristic stuff. The fame went to his head and now he won't shut the hell up and his mouth has got him in a lot of trouble.
Did it all happen overnight? Was there a main trigger event or was it a gradual thing?
Most people point to Musk's tweet calling a rescue diver who saved several children from a flooded cave a "pedo" for turning down his submarine idea. The idea was dumb and calling the guy who just rescued the lives of several children a "pedo" was incredibly bad taste. It was the first inkling that Musk might not be a Tony Stark styled billionaire genius and might just be a rich asshole.
What, exactly happened to twitter after his takeover
Firstly, Musk was forced to purchase Twitter or face a massive fine for stock market manipulation. I don't think he ever actually wanted to purchase Twitter, but Musk has a habit of thinking the rules don't apply to him because he has money.
After the purchase went through he essentially became the King of Twitter and made so many terrible decisions I can't be bothered to list them all here. Just know that he once tweeted that he was walking around Twitter HQ and randomly unplugging servers to see what would happen and also got into arguments over Twitter about how Twitter's systems work...with Twitter's lead engineers...and then fired them for disagreeing with him...even though he was wrong. Oh he also said posting hate speech was fine.
is reddit truly a by-the-book carbon copy of his decisions with API changes?
In an interview, the Reddit CEO has stated that he's using Musk's Twitter decisions as a model for how to run Reddit. Which is terrible considering Twitter's value has plummeted by tens of billions of dollars since Musk took over and has gotten in trouble with major advertisers as well as government organizations for being filled with hate speech, misinformation, and propaganda.