ahhh....well I guess that answer depends on what came first, the chicken or the egg..
IMO, it appeared to me to be a extension of the Arab spring media stories. I think that and years of angst against the Assad regime, was responsible.
I think it was after that that we got involved, ie: Benghazi arms shipments.
If you have not yet noticed, the US CIA and State department is not that smart, plus they are being led by a idiot.
If we got involved at the start - we should have bailed once the FSA proved to be just a front for sectarian jihadis. I get wrestling Syria away from Iran may have been a good payoff but the plan went FUBAR.