User blog comment:Xilinoc/Ch. 575 - THE KILLERS HIGH/@comment-493809-20140402153858/@comment-11051098-20140402163030

It was shown during Nimaiya's speech to Renji about Asauchis that Kenpachi picked up an Asauchi from a dead Shinigami as a kid, before he met Unohana. His case seems to be purely that he always possessed such an intense Reiryoku that he was unable to achieve a sealed form.