Ask HN: Wrote a technical book but risk of getting fired

However, all of the sudden, my manager put me on a PIP, despite explaining that I wrote the book on my free time. Along that conversation, my manager implicitly felt that I was working on the book on my working hours, which is absurd reason or cause for being put on a PIP.

I am performing, and not slacking off in my work. With this experience in mind, does this mean I should use a pseudonym for writing a book in the future?