I believe that Tetsuo was just different to begin with. He was always less important than Kaneda and just couldnt live up to that name. Once he was "awakened" he realized that he had the power to prove he was worth more, but he slowly starts to lose himself to this power. As to why he was so powerful, I can only guess as well as you can.
What i can assume is that his emotions, which were far more magnified than most peoples stored the energy he had begun accumulating when he was a child, possibly around the time that Kaneda had helped him in school with the bullies. He kept his feelings inside of him and wouldnt confront Kaneda or anyone else... After his accident, he opposed Kaneda and was a new person almost.
Thats the best I can do.
