Andy Diggle announced that he would be leaving his upcoming run on Action Comics, a full month before the run was scheduled to start.
Sadly, I’ve decided to walk away from Action Comics for professional reasons. — Andy Diggle (@andydiggle) March 20, 2013
It was the toughest decision I’ve ever had to make, especially with Superman’s 75th anniversary and Man of Steel on the horizon. — Andy Diggle (@andydiggle) March 20, 2013
I wish nothing but the best of luck to whichever new creative team picks up the red boots and cape. — Andy Diggle (@andydiggle) March 20, 2013
Based on these tweets and one where he mentions "the gory details", it appears that this is just the latest casualty of DC editorial controls rubbing a creator the wrong way.
It's unknown whether any of the issues he was solicited to write, Action Comics #19, #20 and #21 will come to pass. UPDATE: Tony Daniel has been announced to be the "Writer/Artist" for upcoming issues. UPDATE 2: Tony Daniel seems surprised on his Facebook to hear that DC announced that he's now the "Writer/Artist" for the rest of the arc.
This, of course, is just the latest in the revolving door that is DC's creative teams.
UPDATE 3: Well, at least DC is taking things in stride. They just promoted Diggle's Green Arrow: Year One softcover on Twitter.