That could very well be! Two really different kinds of writing may be hard to do in the same day. I usually have to take a break between projects if I work more than one in a day, but with nonfiction the difference is bigger. I have some friends who are/have been academic researchers, and who also try to write fiction, and they find it really hard to get into the fiction writing mode after doing academic writing. I imagine it can often be the same with other forms of nonfiction.