Board Thread:Lore Discussion/@comment-97.81.240.58-20130530203219/@comment-24489208-20130602214052

I find legendary dragons very strange, they could very easily revolt against Alduin but they don't, as Ohdaviin says, the dragons are questioning whether Alduin is the strongest but none dare stand against him. It's most likely that Legendary Dragons are just massively powerful cowards who wont revolt against Alduin in fear of death and will only attack when it is extremely necessary due to the power of the target (Explaining why they only appear very late).

Another theory is that they are not so much cowards but wise like Parthuunax, except only they serve Alduin. They wont fight Alduin as they know of the Dragonborns come and will only challenge the Dragonborn when they deem him worthy.