I suspect every writer--or really, artists in every field--have a strong obsessive streak. I certainly do, as Beloved Husband and Beloved Son--and Beloved Mother, come to that--would testify. It's a most useful attribute in working through a novel-length work.
The straightforward map for the book hit a roadblock. Not writer's block, which I don't believe in, but a hairpin curve in the road, to extend the driving metaphor. In the case of this book, the change in the plot is because of a lucky event--my editor wants this book to become a series of three, so I can only rejoice. At other times, a better idea pops up during the work, or a darker and more disastrous plot turn occurs to me, and I decide the book will be better, stronger, more compelling if I use it.
And so--a pivotal scene undergoes a major transformation. I wrote the scene four or five times, then consulted my medical authority (luckily for me, my sister is a nurse) and wrote it again--twice. It's lovely to be obsessive. A smarter writer might skip over that scene and go forward, but my process is too linear for that. I can't go on to what happen next until I fully understand what just happened. Obsessive!