Why is the Hotmail password the accepted password for my husband's user account on Windows 10?

He changed his password to his Hotmail account on our new computer. We had not yet set up user accounts on this new computer. Then when I went to use it (under new user) his name appeared, and it asked for a password to continue! We had no idea that the pw was the new one for his Hotmail account. How'd that happen??

I just tried every password we've ever used for anything, until it worked.