Most people seem to be giving him the benefit of the doubt that he has cold feet. Personally, I think this was sabotage, he had no intention of buying Twitter in the first place and this was just a ploy to fuck with Twitter.
Doesn't really add up, because he's going to be forced to go through with the deal anyway. I haven't heard a single lawyer with relevant expertise suggest otherwise.
Since that's where we're going to end up, it would be much simpler for him to just cleanly buy Twitter, for whatever purpose he intends.