I don't think anyone in the CIA missed anything, I'll bet some Russia analyst has this one nailed months ago and was just ignored.
I think Russia saw an opportunity and pounced. They rightly judged that Assad is the best anyone could hope for in Syria. They also saw that Assad is teetering on winning this war, so they can come in and tip it in Assad's favor, secure the area after a couple of years; everything goes back to the way it was. Russia comes out smelling like a rose, with a big middle finger to the US and our Saudi masters.