Sorry for the lack of response, I've been busy with work.
What do you mean by "suddenly logged out?" You were actively doing something while logged in on the app, and then you suddenly saw the screen change to show you not being logged in?
Exactly
Who or what told you that you had to change your password? Or did you mean you decided to change it, out of precaution?
The latter.
A redacted copy of the Email might be helpful for us to see.
Dear Ferg,
The security of your account is a top priority for Southwest Airlines®. It appears that an unknown person learned and used your valid username and password to access your Rapid Rewards account on Southwest...com. This can happen if you use the same password for multiple online services and one of those services is compromised. To address this, we have deactivated your current password to help protect your account, and you will need to reset your password.
Please visit Southwest.com® and select "Need Help Logging In?" under the Member Log In button to set a new password and regain access to your account. Detailed instructions on how to reset your password are included below. We encourage you to use a strong password. You should not reuse your previous Rapid Rewards account password. As a general rule, you should also avoid using the same password for multiple online accounts.
Note that I use a password manager and my password for the WN site was not reused. I'm beginning to suspect that wiping my phone is probably the reason for this. In any case, clearly it was just me. Thanks for everyone's help!