It could have been a logical decision, something along the lines of "you said you're going to use maternity leave in a casual conversation, why don't you pass that report off now to so and so in case they need to ask you questions about it before you are gone for an extended period".

At my last job I was a contractor and took over a retired employee's desk/duties. She has apparently quit doing any actual real work months before and was just doing a small percentage of her work and not documenting what random bits she had actually been doing, I spent 2 months just trying to sort it out with the agency complaining to my employer that I wasn't doing anything, despite the fact I had literal boxes full of printouts and every surface in my cubicle covered, including most of the floor, with stacks of spreadsheets and records trying to figure out what she'd move from what account to which accounts going through highlighters like crazy highlighting stuff once i was confident a specific line for a specific office for a specific month for a specific bill had in fact been moved out of the field office account into the main GSA account for that type of entry, all while trying to keep up on the incoming stuff.

According to HR, there was a documented history of inappropiate remarks on part of the author's manager. On the basis of available evidence, it's reasonable that those remarks are genuine, too.

Here's a thing - the current legal framework means that manager's dislike to a pregnant woman is basically irrelevant.