User blog comment:San&Ashitaka/TITAN THEORIES/@comment-7532698-20131013023013

Just a guess, but i believe that the serum (You know like the one that Eren's dad gave him) some of it or some varition of it might have been poured into the water supply of the humans inside the walls by allies of Annie, Ryner, and Bert and is slowly turning the people into titans.

I think so because in Chapter 42, Reiner was in a hurry to get out of the walls and in Chapter 50, Ymir wanted to get Christa/Historia out of the walls and she said, 'it'll be hell in the walls, soon'.

By the way, DAVE the word Nephilim also came to my mind when I first saw thw titans. The were said to be children of Angels and Human Women in Christan myth.