While Agents Of S.H.I.E.L.D. has not yet revealed how Agent Coulson returned from the dead, Clark Gregg provided some helpful hints during an interview with Larry King. According to Gregg, he really thought Coulson died in the Avengers until director Joss Whedon gave him a call to tell him that they wanted to bring Coulson back to life for a TV series.
As far as to how Coulson was returning, Whedon explained to Gregg, “You know that Phil Coulson just thinks that he had a close call. But as it is in the comics, there’s a kind of mythology there. There’s a much darker and more interesting story. It’s a mystery. Something much more magical has gone on to bring this guy back to life it seems.”
When pressed for what happened in Tahiti, Gregg said, “Tahiti is a magical place. I keep saying that. It’s where I went to recover after being impaled. And as I say in the pilot, it was a spectacular experience. I had a beautiful physical therapist, and I read Travis McGee novels all day.”
“But every time someone mentions it lately, I seem to go into post-traumatic stress, so I think there’s a little more to Tahiti than meets the eye,” added Gregg.
When asked if he knew the full story as to why Coulson returns, Gregg replied, “I know more than the audience, but less than the writers. I know a little bit. I have some hints. I know from what Joss told me roughly where we’re headed. But a lot of it I don’t know, and I kind of like that. I’m finding it out as the audience does.”
Given the hints that Coulson dropped, it seems very likely that black magic played a role in Coulson's return. We're betting either Scarlett Witch or Doctor Strange was involved. It's also a theory that first ran on Comicbook.com, which seems to be gaining popularity online.