Are you using Sync as this is a possible reason to ask for the Primary Password ?

Using Sync is a possible reason to ask for the Primary Password at startup. This is because the credentials to login to the Sync account are stored in the Password Manager and you need to enter the Primary Password to unlock the passwords and connect/login to the Sync account.

Note that this can also happen if you reopen a page with a login form and you have saved a login for this origin (protocol and hostname).