This happened to 2 of my yahoo accounts also (i have 3 in total). I've not clicked on any spam link. I scanned my pc will full scan setting with both Avast and malwarebytes and both came up clean.
In my situation they seemed to spam email and CC me a copy too. As someone else stated they seem to spam a few email addresses. Some are valid friends emails and some seem to be made-up.
I changed my passwords on both accounts. Nothing since, though unfortunately some of my friends who were on spam list, tried to spam me back.
The commonality of all this does suggest that the yahoo mailing system is the common de-nominator. I suspect that they were compromised on a big scale and instead of telling people, admitting they were at fault, they have kept their head down since nothing happened immediately. Now it seems it as.
Hope they fix it soon. Change password if you can.
Freeserve -> Pipex -> Be