Currently, according to this KB article, there are certain set rules for activity associations:
There are a few exceptions to the above. For example, an exception is that emails sent and calls made from any object record will also be associated to the relevant contacts that were being emailed or called.
These rules are not very intuitive for many users. For example, see this thread https://community.hubspot.com/t5/CRM/Associating-contact-activity-to-specific-deals/m-p/308086#M1171...
It would be awesome if we could have a toggle that allows users to be able to toggle automatic association of all activities for the associated contact/ company, or specific associated deals/ tickets.