Aside from just good plot devices, it could be that the appearance of so many dragons has to do with the events in DA:O. An impending Blight would stir up all sorts of creatures that were in hiding. The dragons in DAII could be residual from that.
Or, the dragons have just always been around, but adventurers and explorers never stumbled upon them - they didn't venture out to the depths or heights where the dragons had taken up residence (as with the High Dragon in the ruined temple in DA:O). And if any brave souls had come across them, they may not have survived the encounters, and therefore the discovery would never be revealed to a greater public.
And maybe the emergence of so many dragons is a sign of some great event to come. A portent of some cataclysm perhaps.
All just theories, mind you. I really lean more towards the plot device/have-to-throw-in-some-tough-fantasy-battles excuse.