A new feature in MSCRM 2015 is the ability to synchronize tasks created in Outlook and assigned to another CRM User. The User who created the Task will remain the Owner of the Task in CRM until the task is accepted by the assigned User. Prior to MSCRM 2015, tasks created in Outlook and assigned to another user were not synched to MSCRM when the task was created.
This feature is only available if you have upgraded to CRM for Outlook 2015 and is turned OFF by default. Enable this feature if your Organization generally creates tasks in Outlook instead of MSCRM. Here's How:
Navigate to Settings>Administration>System Settings. Click on the Synchronization tab, check the "Synchronize tasks that are Assigned in Outlook" check box: