- dead period as September/October are usually slow months;
- end of Splatoon 2's bundles which kept sales higher on a weekly basis;
- end of Octo Expansion effect, which launched a few weeks earlier and definitely pushed back some sales.
#1 In that case, sales would have increased again in 2019 to be closer to the 2018 baseline. They haven't.
#2 When did the bundle start? From the release of the game until exactly end of September 2018? If it is this bundle you are talking about, it started in July 2018 and has no bearing on the much longer period which I base my arguments on.
#3 Seems arbitrary to me, especially as the Octoexpansion did nothing on release for the sales of the base game. Also, it launched in mid-June. The decline happened late September/Early October. Ain't it more far fatched than NSO?