Good question, Sue.
I've actually thought about it too.
I think it's possibly twofold.
One, there was an expansion in interest in California in viognier as a "next big thing". Didn't happen, or at least not nearly to the degree that some wineries/winemakers had hoped it would. So, a lot of viognier to be had on the market.
Chardonnay is still the bear in the market in the US, accounting for roughly one quarter of the wine guzzled in this country.
There was a commanding reason to explore alternatives to the by-then expensive and counter-productive use of oak barrels in the chardonnay, and generally lightening the overall style of chardonnay. All the cheap alternatives were explored----less oak, alternatives to barrels, like the staves, etc.---and so on.
Trouble is, when you get down to it....stainless steel chardonnay, while it can be good, also can be soulless, bland, primary and supremely dull and boring all by itself, unless done carefully and in the hands of a very good winemaker.
So, one of the alternatives that was explored was the blending of relatively expensive but not all that compelling Chardonnay with the suddenly available and not as expensive Viognier that added a wonderful floral component and soft aromatic fragrance and silky texture to either dull or shrill Chardonnay.
And it turned out to be a simpatico blending----as with Semillon and Sauvignon Blanc, say---with the combination of the two often resulting in a better wine than either would be separately.
Now that Viognier is going through a mild resurgence and the abundance in the open market is not as much as it once was, and the price isn't as cheap as it once was, the paradigm might not be as compelling...on the other hand, now that the concept blend is established, consumer demand might keep it thriving. That, and the fact that a tiny little bit of Viognier can go an awfully long way in a big vat of Chardonnya; it doesn't take much.
Anyway, that's what I think happened.
Well, that, and the fact the Aussies are crazy about aromatic grapes like Viognier and also crazy about inspirational (and sometimes seemingly weird) blends of wines. Which I like too.