User blog comment:Speculation no Kyojin/Episode 25 - The Titan in the wall/@comment-82.201.244.79-20140726004354

I think that the walls are titans shifters who transformed into stone with or without their will like what Annie does when she wants to protect an area like her neck she transform it into diamon so the titans just transformed into stone and after annie desteroyed the outer surface of this stone and they were in something like a sleep like she did and now after Annie broke the outer surface they waked up

P.S(I didnt read the manga)