Board Thread:Manga/@comment-26056057-20160509020829/@comment-26932251-20160511045745

Zeke references having other 'warriors'. I think it's a safe bet that the humans who were able to be ordered to stand in a very long semi-circle and take the titan injections all at once are the aforementioned 'warriors'.

Occam's razor, simplest possible explanation is the one that should be accepted. I think it's more likely that Zeke and Reiner simply killed all of the titans that were nearby. Remember that the titans that passed through Shiganshina didn't stay in Shiganshina. The "Smiling Titan" for example was somewhere just out in a plain far away from Shiganshina so it seems like the natural behavior of dumb dumbs is that it's kind of like they diffuse into a vacuum. Shiganshina and the lands between Wall Maria and Wall Rose was the vacuum when the gates were breached and they just sort of poured through. Long story short, there didn't have to be that many dumb dumbs there in the first place before Zeke got rid of them.