So, in HTTYD2 we learn about Alphas. This a pretty new aspect of the HTTYD universe, but an aspect that we did get to see in the first film. The Red Death was a "queen", a dragon that ruled over a particular nest. How does a dragon become a queen? That I do not know, but I imagine it has to do with the dragon's size and ability to get other species to do what it wants. A queen seems to also be able to, to a very, very small degree, bend the dragons that live in its nest to its will. We see the Red Death do that when it seems to signal Toothless to return, sees he has brought nothing, and tries to eat him. At the time of the first film, I wasn't sure if that noise really had any kind of controlling affect, but after seeing the second one it is obvious, queens can manipulate dragons to their will. They can't do it much though... the Red Death also used it's size to terrorize the smaller dragons into obedience.
And that is why what Toothless did was so massive, He challenged the Alpha. At first, this HAD to be a one on one duel. No help from the outside. Everything inside of Toothless's dragonish nature must have screamed out against this. Red Deaths, Bewilderbeasts, they are Alpha material. They are large enough to defeat each other, but one lone, tiny, flightless Night Fury, even if it were to win, how would he ever effectively wield that much authority? Little dragons DON'T become Alpha. That's what titans do, and when they get the role they keep it for decades to say the least. But Toothless was so furious, so passionate, so protective, and so vengeful, he broke every code. He was gonna fight the system. If he couldn't resist the Alpha in order to protect Hiccup, he was going to BECOME Alpha, or he was going to die trying. When the other dragons saw the challenger (Toothless) beginning to prevail, the force of the Alpha's control was weakened. He was no longer secure in his reign over them, and now it was anyone's game. Who would become Alpha? Toothless or Drago's Bewilderbeast? As soon as it got to that tipping point, the dragons saw the truth. Toothless was the Alpha they needed, the Alpha who would protect them, and since it was anyone's game on who was going to be Alpha, since a challenger had arisen and at that point was winning, the swarms flooded to Toothless's side. Toothless changed the world of dragons forever, and he did it to insure that Hiccup would be safe and that the dragons would be free to live and love again. It was an incredible moment! I still get chill bumps thinking of how enraged and courageous that little black dragon was.