Even before Kofi Kingston won the WWE Championship, there were speculations regarding Big E's heel turn. It didn't happen because Kofi's win was a greater moment for the New Day and there was no logical reason for Big E to turn heel at that moment.
But after Big E's injury, the rest of the New Day recruited Kevin Owens in his place on the very next episode.
The aforementioned could be the reason for Big E to turn on his stablemates. He would have a legitimate reason to attack his former friends.
This is the most obvious reason for Kevin Owens to join the New Day. Kevin Owens is a fighter, a no-nonsense wrestler whose character is totally opposite from that of the New Day.
Owens danced and ate pancakes, and did full justice to his role as a New Day member, but there's something more developing beneath the surface.
Owens could turn on the New Day when they would least expect it. This could bring him back to the title picture and would re-establish him as a top heel.