Windows 11 - Outlook instance is already running when computer starts up.

When Windows boots up, sometimes I can't open Outlook 365. Typically, ending the process and restarting Outlook works. I started checking the task manager immediately on boot up, and there are times when Outlook is already running and times when it is not. If Outlook is already running, it isn't possible to open Outlook unless the process is ended. If Outlook is not running, there are no problems. 

Is there a reason Outlook is sometimes already running when first booting up Windows? I tried searching for an answer but didn't get very far; a lot on disabling add-ons and resetting the nav pane, which doesn't apply to my issue.

I did run a repair, and it didn't fix this issue.

Thanks for your help.