Ofc it was because of the humans. It starting to make more and more sense. It feels like the story is finally starting too. Likely that the white dragon died not because he was too weak and made a mistake too. The cliché would have been he? died trying to protect something, let's say a younger dragon/his child or a egg. Well at least by the way those "dragon slayers" have been talking about the whole thing, it's highly likely that some x factor made him lose to them.
I hope Kalsarin will be able to transform back into a dragon soon too.