Why is Windows 10 making me sign into my account when I never askd to be signed out? Something changed by itself. How can I undo the change?

If you can't stay signed in your browser isn't storing cookies from Yahoo. You have to change your browser settings to allow them.

To enable cookies in Firefox: https://support.mozilla.org/en-US/kb/enable-and-disable-cookies-website-preferences

To enable cookies in Chrome: https://support.google.com/accounts/answer/61416?hl=en

To enable cookies in Edge: http://www.windows10update.com/2015/07/windows-10-tutorials-128-allow-or-block-cookies-in-microsoft-edge/