Board Thread:Theories/@comment-30473998-20170816133851/@comment-32443384-20180204075849

I think there is just a random chance of any given titan somehow becoming an abnormal.

even pure titans vary greatly in size and behavior, most prefer to grab with their hands softly and bring to their mouths to eat, but a titan is still a pure titan, not abnormal, if it eats humans directly with its mouth, as the titan that leans down on all fours and grabs a soldier from his horse.

every human eldian will react somewhat differently to the titan serum, and some will just naturally become abnormals for one reason or another.

in rod reiss case I think it was a combination of natural predisposition, royal blood, the serum being eaten/licked rather than injected, etc. that lead to his becoming abnormal. on the other hand the titan that mistook Ilse for ymir was apart of a large group of people turned into titans all at once and it was the only one that came out very abnormal. same again for connies mother, all other humans in the area became normal despite no great difference in method of creation.

edit: I am really curious about this as well, I'm sure in-universe there were lots of Marleyan scientists from the Titan Biology Research Society experimenting trying to get controllable titans, or deadlier titans.