If you don’t like that, and want your default account to be selected for all new emails, you can do this via a Registry setting in Outlook 2010 or an Option setting in Outlook 2013, 2016, 2019 and Outlook as part of a Microsoft 365 subscription. When you have multiple accounts configured, Outlook will automatically set the “From” account based on the mailbox folder that you are in. When I create a new message, it automatically selects the sending account based on the folder that I'm currently in, instead of using the default configured account.

I have multiple accounts configured in Outlook.