I don't see why there are so many hates with the ending. To be fair, it was kinda unexpected, but not abrupt or absurd. My point is, Dragon Head might take place in an ongoing-apocalypse (or I like to call it "Ultra Disaster"), but I guess some people mistook it to be a horror-survival manga. There were elements of a horror-survival here and there, but it was *not* Dragon Head's main theme, the main theme was about people's fear and how they dealt with it when the world fell apart, and to that it did a good job until the very end.
What exactly was the cause of all those chaos? Nobody knows for certain and it should stay that way. The disaster in Dragon Head was too huge and bizzare for anyone, even the best scientists in the world, to figure out in a glance, and hence any attempt allowing a group of young kids to miraculously identified the real cause would just feel forced. And I'm glad that all we got for the ending were wildest guesses, because it was cool. "Finding an explanation" was the very first thing that pop up in everyone's head, but as the journey go along, actually it was to explore human fear that was the main purpose of Dragon Head. I guess some people was into Dragon Head 'til the end just because pure curiosity, and the fact that no clear explanation was pulled off made them pissed, but to me, it was so realistic, so on-point and somewhat satisfied. It was a long, brutal and tiresome journey, so at the end, just to be with your love one and watch the world around you burst into chunk of flame was such a relief (It kinda remind me of "Fight Club" ending).
Again, if you somewhat hate the ending, it may just because you totally mistook the manga and what exactly the author were trying to portrait. What about Japan after that? What about our world? What was the cause? They were not told in the end because (I guess) they were never the real intention (exact opposite of how careful fear was discussed and defined), so if it still does not click for you, just use your imagination and pick your favorable scenario. For me, I prefer some questions to be left unanswered like this, because it was just something true about life.
I didn't find the characters in Dragon Head all that amazing but the manga captured the atmosphere of a ruined world so brilliantly, I kinda change the way I see our human world a bit.