Classic Dan, to resign like that. Not a whisper to anyone, then ambush the media and your enemies in the parliament – on your own side as well as the Coalition.
Why was it so sudden? Probably catching factional foes unprepared, in order to ensure that his deputy, Jacinta Allan, gets to be “next premier”. There were some internal rumblings, attempts to deny him his choice of successor, and it is classic Dan to catch the plotters off guard.