User blog comment:Speculation no Kyojin/Episode 25 - The Titan in the wall/@comment-7418318-20130929164007

That the Titans are only dormant is even said and shown outright in the manga.

When it was exposed to sunlight, the eye slowly turned toward Mikasa. That, Pastor Nick's insistance to let no sunlight get to it and Hanji's research together show that Titans deprived of sunlight go into a kind of suspended animation, but don't die. And they become active again once they get some sunlight.

Maybe the Titans were "living weapons" that were employed by the countries of the world until something happened to cause them to go out of control and those remaining in control of themselfs were ordered to "become" the wall, or maybe volunteered. They might also have been led to believe (whether truthfully or as a lie) that the walls are only there for giving the country a chance to build up their forces again and that they'd be brought out again for the counter attack.

If that was the case, the Homeland of the hostile Titan Shifters might have been founded by shifters who deserted becaues they didn't want to become part of the wall, no matter how short it would be. It could also mean that they inherited this trait from their parents, unlike Eren who most likely got it from his fathers injection.