I personally think that humans can be transformed into titans. I came up with this theory because a titan doesn't need nourishment and it seems all they need to live is sunlight. The idea hit me when I watched an episode where someone stated that because a titan didn't have a digestive system, they simply regurgitates them (it's really scary) and the victims are all covered in this weird looking goo. So maybe the titans do this for reproduction. I still don't know what caused this, but it's just a theory.
Also when I read about the incident in Ragako, Connie's hometown, the buildings were destroyed, but there's no blood or bodies, the horses are still in the stable, and they found a titan that looks like Connie's mom.
Well, that's my theory and my personal opinion. I just discovered this anime and started watching it 3 days ago.