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As much as the pedant in me may find the word "unprecedented" to be wildly overused, it's reasonably clear to me what the news media is driving the definition of the word away from genuinely "having no precedent," which is how Merriam-Webster online currently defines the term, to something more akin to "unusual (perhaps highly so) in the current cultural context." Used in this way, "unprecedented" can be thought of as a form of neologism, although semantic shift is the proper name of the process taking place.

And this shift seems to be driven by the fact that in modern American English, "unusual," is a fairly mild world, since it can refer to anything that lies outside of the speaker's experience, even if it it commonplace for others. And so "unprecedented" is being moved into the open space left by the fact that there isn't word that means "(highly) unusual to more or less everyone." Or, at least, the speaker believes that it's unusual to their entire audience.

And it makes sense that this shift would be driven by the news media. Time is often at a premium, hyperbole often comes with the perception of genuine importance and there wasn't another readily-accessible word to be used. Interestingly enough, it fits the precedent.