IGN interview with Rucka
[...] [The Avengers] declare war on [Frank Castle].
They notice him and they can't ignore him. It really is as simple as that. Something has occurred and they cannot in good conscience go, "You know what? We’ll just pretend that’s not happening and continue on our merry way." Something has happened and they have to respond.
[...] I think Frank knows that any conflict with the Avengers only ends one way. There is a certain tactical awareness that he has that makes him not as easy a target as they think he’s going to be. [...]
[...] This isn't as simple for the Avengers as going, "We've got to stop him." How do you stop him? How do you stop him short of killing him? That's a problem they're going to have to deal with.
[...] At some point a few months ago I was informed that Marvel wanted to end the book and would be spinning Punisher out into a different team book and I wouldn’t be writing it. So [Steve] Wacker and I talked about how we were going to wrap this up. [...]
[Rachel Cole-Alves] is absolutely integral to the events, yeah.
[No Marvel Universe work] planned beyond War Zone.
Much more, of course, at the link.