How are Tasks Assigned by default?

Tasks in Salesforce are by default Assigned to the Record Owner of the Account. Any Objects which have a Master-Detail relationship to the Account (Sales Opportunity, Sales Appointment, Quote and Sale) will default to the Record Owner. Note the Objects which have a Master-Detail to the Account in the Object Relationship Diagram. 

Keep in mind, if the Account was created via an eLead integration, the Record Owner is most likely the eLead User. This means when you create a Task from any of the Objects with Master-Detail to Account, the Task will be Assigned to the eLead User by default. 

In order to re-Assign Tasks to a Group of Users, we recommend creating a User Queue, adding your Users to the Queue and then Re-Assign Tasks to the User Queue. 

A Flow automation can be used to re-Assign Tasks to the User Queue anytime the Assigned To User is a specific Profile or a specific License Type, like an Integration User.