DFP is the publisher adserver for nearly all of the web. It's a very stable, feature-rich technology at a reasonable price. Being the publisher adserver also allows Google to have a strategic tag-on-page relationship with a high percent of publishers. This led to dynamic allocation - in which Google could route publisher impressions directly for resale in AdX when a publisher's directly sold campaigns were pacing well enough. This, in turn, allowed the publisher to achieve higher monetization while making AdX the preferred exchange. Naturally, folks like Rubicon and OpenX (really, all other SSPs, exchanges and networks) were not thrilled - their only option was to have DFP estimate their yield, which was not very accurate and undermined their value prop.
Through some clever hacking of DFP, SSPs created header bidding - where the dynamic valuation of an impression could be passed into DFP as a line item. This meant publishers could now have multiple exchanges competing for a single impression. For a variety of reasons, this wasn't the ideal outcome for Google. While it didn't fundamentally challenge DFP's prominence, its competitors certainly highlighted Google's reluctance to support header bidding. Further, the more header bidding partners a publisher added to the page, the worse the publisher's user experience became, and the worse DFP appeared as a tool. But ultimately, there was a reality that Google simply couldn't lock everyone out of DFP now that they'd found at least one way in.
As a result of the massive industry support of header bidding, Google recently opened DFP to third party server-to-server demand in DFP. This means that, as part of the dynamic allocation, Google will now make server-to-server calls to find the SSP/exchange that provides the highest monetization. This is a server-to-server call, rather than a client-to-server call in header bidding. So theoretically it's more efficient for an end user because the browser makes fewer calls - and easier for a publisher to implement, because it doesn't have to worry about wrapper tags and changing code on the page. This is a very new program and there are only a couple beta partners.
That said, there are many in the industry that remain suspicious of Google's motives here or dubious of the product. Rubicon, for example, said "We see our partnership with Google as complementary to innovations in header bidding" - meaning this certainly won't stop their innovations in the header bidding space, having noted that it doesn't include support for deals at the moment. AppNexus, which is now largely in the business of being the anti-Google, said the following, "[i]f Google has finally embraced open dynamic allocation, that’s a good thing for publishers, and we welcome them to the party. If they’re providing tools to help publishers see exactly what’s happening inside their auctions, all the better. If, on the other hand, this proves to be just another measure to create artificial advantages and disadvantages in the market, then it’s the same old game."
So we'll see how this net out, but it seems like this is good news. Header bidding definitely isn't dead, and will likely keep growing for a while - but it appears that there are more and more interesting ways of helping publishers effectively monetize their inventory, which means more opportunity for TripleLift.