He said he’d do it. He paid them to begin the process. He then tried to back out with a variety of tactics, including trying to say that Twitter was mostly bots.

The Twitter lawyers forced him to complete the sale because he has no valid reasons to cancel the contract.

https://www.nbcnews.com/business/business-news/twitter-elon-musk-timeline-what-happened-so-far-rcna57532

This is a pretty good overall timeline.