Why do I keep getting a pop up wanting me to enter my password for my yahoo email?

This is from yahoo's page

App access and security

Yahoo has a variety of ways for you to maintain your account security. Some third party email applications use an older security protocol to sign in to your account. Find out if you’re using an app that fits this description and how to improve your app security.

Determine if you’re using outdated or less secure sign in

• Blocked access - We may block sign in attempts from older apps and you won’t be able to access your Yahoo Mail.
• Email notices - You may receive emails from Yahoo if we detect sign in attempts from apps with outdated sign in security. If you don’t update your security, you’ll continue to receive these notices.
• Common email apps using older security protocols for sign in - You're using the following:
  • Outlook.com and Outlook desktop app for Windows and Mac
  • Apple Mail on Mac OS 10.9 or lower
  • Apple Mail on iOS 8 or lower
  • Outlook Express

More securely access Yahoo Mail and stop email notices

• Use our app - Use the Yahoo Mail app for Android and iOS or https://mail.yahoo.com.
• Use a third party email app with modern sign in security - Like the Mail app on iOS 9 or above for iPhone, or the newest, verified Gmail or Outlook mobile apps.
• Review email applications that access Yahoo Mail on your computer and mobile devices, and remove any saved Yahoo Mail passwords from apps you no longer use - Email apps that save your passwords may regularly fetch new email data, which can trigger the email notice.

Temporarily allow or deny access to apps using older security sign in

While we don’t recommend granting access to apps that use a less secure sign in method, you can temporarily choose to allow access to them for the time being.

1. Go to your "Account security" settings.
2. Select Allow apps that use less secure sign in.
3. To deny or turn off app access, deselect the undesired app.

It may be something yahoo has done. I know I get a weekly reminder from them to. 'turn off insecure access', which is translated as 'anything but our app'. Since I access mine both via the web on my laptop and via an app, I see this on the browser version and not on the app version.

Have you tried to go into the settings, mail, and enter your password there? Or to remove then re-add your mail info?

If you read around, Yahoo has a "bit" of a security issue these days.

I would suggest going to their web portal and accessing your mail there. They may be requiring a password change right now. They likely have no way of "saying" that to you on your iPad.