it's mostly a fan theory but they've been teasing skyes background for awhile now.
With "A Hen in the Wolf House," "Marvel's Agents of S.H.I.E.L.D." dropped some major clues for the direction of this season. In a dramatic confrontation between Skye and Coulson, he suggested that she may not be entirely human -- that she may, in fact, be an alien. And, well, he certainly isn't wrong. Expounding on my previous theory, here's a few more clues that point to "Inhumans" taking center stage in the Marvel Universe