Recently Microsoft made the decision to force 44 million accounts to do a password reset. This wasn’t the result of a breach or a compromise of their systems, but rather a proactive move.
What Microsoft did is compare their accounts to those that have been compromised through other breaches and determined that there might be some password sharing going on.
You know what we say here at CU*Secure: Don’t re-use passwords.
And no, we don’t expect you to remember unique passwords for every site. We recommend you use a password manager. For more information about these Microsoft emails and how to set up password managers, check out this article on Lifehacker.com.