It may come as a surprise, but there are several reasons for this annoying occurrence. One that's a common reason for windows security sign in pop up Windows 10 is HomeGroup sharing. You'll need to disable the password security if you're utilizing HomeGroup sharing to connect many PCs. To do so, go to the Windows Search box and type "advanced sharing," then click "Manage advanced sharing options." After that, expand "All networks." At the bottom of the page, click "Turn off password-protected sharing." Then exit after saving your modifications.