This Windows security popup issue usually occurs if the device you're using hasn't been verified for the Microsoft Account you're using or if you have Apps that require a verified account to access data, such as settings sync, Mail, and so on. The other answer to why does my Windows security keep popping up is there's a problem with your Windows Credential Manager, such as an incorrect entry with an old password or if it's been disabled. You must reset your system Network settings to stop or disable security popups on Windows.