Nokia has been aggressive about releasing new Lumia variants -- typically about every six months. I think the reason this is appears to be coming out after just 4 or so months has to do with Windows 8.1. It appears that Windows Phone 8.1 will support more hardware and greater screen resolutions, and the 1520 seems to be designed to take advantage of this, as the first phone to ship with 8.1.
Beyond that, I'm not sure this is truly a 1020 replacement. First, the 1520 is clearly a phablet -- and many users do not want something that size. Secondly, while it will have a better processor and screen resolution, rumor is that it will have a "normal" camera and not the 1020s 41MP camera.
I don't see the 1520 actually being the "flagship", rather I believe it will play a role more like what the Samsung Note 2 does compared to the Galaxy S4 -- especially since new version of the Note typically comes out opposite the release of the new version of the Galaxy S.