Spam and spam fighting is a really complex game of cat and mouse. Spammers are constantly trying to find their way into your inbox. I have a few ideas of what could possibly have happened in your situation.
Overloads, hacks & tricks
Spam filtering services can sometimes get overloaded and as a result, choose not to filter at all. Rather than delay or deny what might be legitimate email, they elect instead to simply deliver it, even if they can’t run their spam analysis on it. This doesn’t happen often, but when it does, it manifests as a sudden influx of spam.
Also, accounts can get hacked. You said that the spam can somehow “authenticate” now. Authentication means that they can login as a Yahoo! user. One way that can happen is if spammers have hacked someone’s Yahoo account. Now they can use that account to send spam, even spam that looks like it comes from a non-Yahoo address.
As I said, it’s a game of cat and mouse. Spammers are constantly looking for new tricks and new techniques that will bypass existing spam filters. They may just have found a new one. If that’s the case, I would expect the spam filter to continue the game of cat and mouse by eventually catching up: figuring out the loophole, and plugging it to stem the tide.
Using the junk or spam button on email in your inbox that’s obviously spam is, in fact, one way you can help.
You’re on a new list
Here’s another possibility: you may have just gotten on to a new spammer’s list.
Spammers can make money by selling their lists to one another. If your name was on one such list, you may now be getting spam from a completely new source that isn’t yet recognized by the spam filter. Once again, I’d expect the spam filter to eventually catch up, and again, using the junk or spam button on that kind of email can help speed up the process just a little.
Since there’s zero absolute accountability in email (meaning that there’s no-one checking that you are who you say you are), it’s nearly impossible to stop all spam. As you’ve seen, this kind of thing can come in waves.
The best thing that you can do is to simply use the spam or junk button on spam emails in your inbox… and just not get too worked up about it.