We don't really know what created the Hollowfication process. To quote from Wikia:
It is unclear how the other Visored gained their current status. What is known is Sōsuke Aizen was instrumental in the Hollowfication process, though whether he knew of the full potential of the results is left to be questioned. The process used seems to spread as some form of possible infection through wounds which has the desired effects on souls with high-level Reiryoku, as compared to souls with low-level Reiryoku. The infected experience great sickness and fatigue, eventually vomiting up Reishi in the form of a white liquid, which forms the beginnings of their Hollow mask before proceeding to transform their bodies and warp their minds. Other than that, all that is known is the process is accelerated by increased stress. The conclusive result makes a Shinigami into a primarily Hollow-like humanoid, which is a highly unstable transformation causing mindlessness and aggression.
It should be noted that all members of the Visored except Shinji and Hiyori were cut by Tousen, and Hiyori was wounded by Hollowfied Kensei and Shinji was cut by Hollowfied Hiyori, so Wikia's claim that the Hollowfication process is probably caused by infection through wounds is sound.
However, it is almost certain that the source of the Hollowfication process is the Hougyoku:
Hollowfication/Arrancarification: While not truly an ability of the Hōgyoku itself, the orb took on this property because it is what Kisuke Urahara desired when he invented it. Through this power, the Hōgyoku could be used to bypass the barrier which separates Shinigami and Hollows. As such, it can cause Shinigami to undergo Hollowfication and Hollows to undergo Arrancarification. These processes lead to the creation of a Hollow-Shinigami hybrid and an Arrancar, respectively.
