I think that because of the upgrade ALL the notifications are enabled, particularly the email notifications by default that were missing on the 1.x version OR that are new. We may keep them off or follow the same pattern from the 1.x version.
Can you clarify what notifications you're getting that you didn't expect?
Specifically, prior to the upgrade, you had the setting enabled to automatically watch and receive emails for any content you interact with. That setting was maintained when we upgraded. Thus, any post you make in a thread would have you start watching it. The same thing would've happened in XF1.
(I see you've since changed the interaction option; in XF1, there was no distinction between content you create vs content you interact with.)