Why is it suddenly raining dragons?

The Base Skyrim game has a bug in that the random encounter dragons respawn but they do not reset. This means that the corpse gets moved to the random encounter position without being resurrected, and you either get a live dragon with no loot / dragon soul, or a dead dragon corpse instead of an actual, you know, dragon.

While there's no way to handle this in the base game, this is one of the many things fixed by the Unofficial Skyrim Patch, which you can download and apply through Steam.

Fixed some encounter dragons not burning up nor giving the player a soul when slain (respawned dragons were not being properly reset)

If you're playing on the xbox or ps3, however, you're out of luck.