How did this happen?

Apparently I am now writing an extended synopsis for Dog of the Dead, along with a list of characters both major and minor, and a brief description of the various settings, in preparation for tackling the manuscript of Mortal Affairs (the second book in the “Ms. Whitaker’s Last Year” series). I’m three chapters in. I don’t know quite how that happened. I had intended to go through my extensive notes for Shadow Practice instead (the fourth in “The Ways of Magic”), in preparation for revising the current version.

Does this happen to other people? I would hate to be my editor.