Ah man I've been looking forward to one of the games taking place during an Akaviri invasion. They've been hinting at it since Morrowind and definately set the ground work in Oblivion. I mean the last Akaviri invasion had attempted to enter Cyrodil through the mountains from Skyrim - that was heavily featured in part of the main plot. They also made a point of featuring the influence of the Tsaesci (vampiric serpent race) on the Blades and the Imperial tradition; for example the blades armor and much of the importance of the dragon symbolism come from the time where the Tsaesci had conquered much of the continent.
The clearest evidence that this installment is about an invasion from Akavir (by one or more of the four distinct Akaviri races/empires) is that the symbol chosen for the game is the symbol the Elder Scrolls games have always used for Akavir. People mistake that as just a symbol of the dragon, Talos, or Akatosh because it is so prevelant in Imperial regalia; this prevelance is because of the influence of Akaviri conquerers though and it is actually the symbol used by one or more of the Akaviri empires as well. It is referred to officially as the "akaviri sigil" or "akaviri banner." It's all over Blades-related items since they styled all their weapons, armor, and traditions after the snake people of Akavir.
Also some of the Akaviri civilizations had powerful leaders who had been able to ascend to a dragon form. While the Tsaesci revered dragons's power they did not worship them but rather coveted their power. They enslaved the red dragons and even went so far as to start eating them as part of an attempt to gain their power (not so misguided as it sounds since they were after-all vampiric).
The Ka Po' Tun (Tiger people - they look like tiger-faced man-beasts) on the other hand had a different relationship with dragons. They revered the dragons as well as their power and sought to ascend and become dragons themselves - though not by eating them. The black dragons fled to their lands and they went to war with the snake people; the result was the supposed extinction of dragons from Akavir and a weakening of both empires. Still at least one Ka Po' Tun managed to assume the form and power of a dragon (reputedly the most powerful ever seen); he was described as a black dragon with flecks of golden-orange scales. Since that war this ascension has been the aspiration of many powerful members of this empire.
Where exactly the dragons have come from is not entirely clear although there are a couple of lore-consistent possibilities. It might make sense that not ALL the dragons went extinct in Akavir. Since red dragons were enslaved and bred they may have been able to repopulate them. It's also possible that the story of their extinction was exaggerated by the tiger people to protect the last remaining black dragons (which could have obviously bred over the centuries since they supposedly died out.) Remember also that many Akavir aspire to become dragons themselves through arcane, spiritual, or some monsterous means.
Maybe the dragons of Tamriel had been hiding and are in league with the one or more of the Akaviri. Also, if dragons did reappear or reveal themselves in Tamriel that would actually cause an Akaviri invasion. The snake people went to war to try to enslave and eat the black dragons after all. Finding out they weren't extinct and that they could have an edge over their long time rivals (the Tiger-Dragon empire as it is called) it would make sense that they would invade Tamriel again simply because there were dragons there.
I don't think the plot is going to be quite as simple as "dragons attack, oh noes!" like some people seem to be getting it. Yes there is a dragon in the intro and it is obviously important that they are not quite as extinct or near exctinct as people thought. At the same time the symbol chosen to represent this game (on the elder scrolls website as well as the end of the trailer) is not just a dragon symbol nor is it just the symbol of Akatosh. It is the Akaviri war banner - and they have invaded Skyrim in the past and were eventually defeated. That fits with the whole defeat was merely a delay reference - as well as a lot of the references in Oblivion.
Heck they even went so far as to mention in Oblivion that the Nevarine had gone to Akavir and was not heard from again. Do you not think someone as ridiculously powerful as the Nevarine may have given the Akaviri empires reason to pay attention to Tamriel once again?