I am wondering what event exactly would cause these auto posts to be created, and where should I look for the source of this?
I guess you have imported many solutions/customization in your CRM, However, here is a work around:
Go to settings>>Customizations>>Customize the system>>SDK Messages
You will see that there are multiple auto-posts plugins enabled for same event on same entity, disable them & let only one enabled:
Make sure to Vote as Helpful and Mark As Answer ,if you get answer of your question.
Do not forget to click on Publish button after that