Two facts to consider: 1) Most baptisms take place during assemblies and conventions; 2) the theocratic year goes from September to August.
My theory: the Borg changed the calendar of conventions and assemblies so to concentrate the baptisms in a single theocratic year. E.g. if some conventions used to take place in July/August, for the 2018 series they moved them to September. That way they could count the baptisms for those conventions with the 2019 numbers. Then, for the 2019 series they move them back to July/August.
Of course, this will make numbers go down for the prior and the following year. However, my thought is that they prefer some peaks here and there so they can brag about and, maybe, create some enthusiasm so baptisms increase naturally.
What do you think?