Ars now says that Musk is claiming to have personally scrubbed the otherwise "green" launch due to irregularities he spotted in a control system for the upper stage. Not sure we are getting any full story yet. Not sure he has full a story yet to give.
https://arstechnica.com/science/2017/02/behind-the-scenes-of...
Does he literally have a finger on an abort button? I find that very hard to believe.