To Sleep, Perchance to Dream - sci-fi/fantasy dragon riders

After Earth became uninhabitable, humanity sent six ships to the planet of Altayr. Five of the ships never arrived. The sixth hit atmosphere and broke into pieces. Those who survived the crash were terrorized by the behemoth aliens native to Altayr.

Until people started to find strange orbs in the forest--dragon eggs. These dragon riders banded together to form the Drakonrhedi, the only people capable of killing behemoths. But dissent is rising in the Drakonrhedi: some only want to fight for themselves; others think they should protect humanity.

And others start to question: who gave them this magic? And what do they want?