Why Did Cayde Become an Exo?

Cayde quickly agreed not wanting to saddle his family with his debt and knowing it would one day get him killed. As a result Cayde was turned into an Exo as part of Bray's research.

Did Cayde make up ace?

None of his descriptions of him (such as calling him a "firecracker" in lore entry "All In") are really true but were completely fabricated by Cayde himself.

Did Exos used to be human?

Unlike mass-produced robotic frames, Exos are human minds uploaded in artificial bodies; with the notable exception of Felwinter, each and every Exo used to be a human being.