Thread has been gone for hundreds of Turns. The Holds are tired of tithing, and no one *really* believes that Thread will return - actually, a scattered handful of dragonriders do. There are some in every Weyr, and their dissent is slowly tearing the Weyrs apart while the Holders seek to lighten their tithing duties. Tensions strained to the breaking point, a decision is reached. The dissenters may leave to form their own Weyr, far away - little more than exile. The increased unity among the rational dragonriders would then allow the Weyrs to face the Holder threat.
The only problem is that, bare days after the exiles leave, plague breaks out among the northern dragons. It spreads quickly, and none recover - but none die, and worries settle down as the disease proves to be little more than an annoyance... at least until an opportunistic parasite takes hold and wipes out each and every dragon infected with the initial disease. The handful of dragons not infected by the first disease quietly retire from the northern continent before the Holders have time to react - and besides, the only queen dragon left on Pern has gone with the exiles.
Weyr Under Sand
Late Fourth Interval (long) AU. Dragonriders have been exiled for believing in Thread, but just in time to avoid a plague.