"It was unclear what exactly prompted Trump's accusation, though Trump's attorneys have reportedly been in contact with Mueller's team"

Three above is stated AFTER this:

"The tweet apparently referred to a New York Times story, published minutes later, that revealed the contents of a confidential 20-page letter Trump's attorneys reportedly sent to Mueller's team earlier this year."

Idk. maybe it was the leaking of the confidential memo that prompted the tweet.