According to this interview, Burtt spent so much time doing pre-visualization stuff that in many cases (the speeder chase in AOTC, for example) those early assemblies would be completely his to play with. Then, he'd sit down with Lucas in the editing room, where Lucas (of course) always had veto power over anything. But the point here is that many of the high-action sequences rested initially within the mind's eye of two people: Lucas & Burtt.

At some point on AOTC, Burtt's pre-viz work just naturally bled over into him co-editing the film. And I find it amusing that on AOTC, after Burtt had done his editing of the film, it was only called a 'first assembly'. It wasn't called a first cut until Lucas had combed through it, although Lucas once again gets no credit as an editor.