I was thinking that perhaps what happens inside the raid instance there affects how and when they spawn. You know how there are loads of eggs inside on each side of Onyzia, what happens if you try to spawn as many of those eggs as you can before you die, then go out to check.
My personal experience is that I did one side of eggs, then healed myself while running out with 1001 whelps on my tail and Onyxia, then when I came out, I circled the mountain, and found 3 Spawn of Onyxia.
I do know dungeons and outside worlds arent usually connected, but I got the idea because when a dragon is killed, there are also displayed a dragons head in Orgrimmar, so they do have ways of connecting events inside dungeons to the outside world, even if they might be happening on different servers.
After doing it once, I tried again, but saw no spawns when I got out. So perhaps it was coincidence the first time, or perhaps someone else snatched/checked+left battle before I got to check the area the second time.
It is just an idea, but I would love to hear from others who have the chance to team up with someone to see what happens outside when the eggs/whelps are awakened inside.
Oh, and on a sidenote, I read somewhere that they have a 10min spawn timer, which doesnt compute with my experience, since I checked the area before I went inside, and when I got out, less than 10 minutes later, I saw 3 spawns, so the rumor that its one spawn every 10min, is not correct. Atleast not without exceptions