Looks like someone has attempted to hack Yahoo mail. Yahoo has this to say via their tumblr blog.
“Based on our current findings, the list of usernames and passwords that were used to execute the attack was likely collected from a third-party database compromise. We have no evidence that they were obtained directly from Yahoo’s systems. Our ongoing investigation shows that malicious computer software used the list of usernames and passwords to access Yahoo Mail accounts. The information sought in the attack seems to be names and email addresses from the affected accounts’ most recent sent emails.”
So if you are trying to log in to your yahoo mail and get a prompt to reset your password, this is probably why.
read the full blog post.