A combination of factors, actually. First, wordbuilding turned out to be more detailed than I'd planned, though I think I only put things that are relevant for the story. Second, I mostly discovery-write, so subplots certainly show up with enviable regularity. Third, I underestimated the complexity of the plot with 4 POVs and multiple magic systems.
But, at least, it is a whole lot of fun despite all the surprises