I think a lot of it had to do with Joe's personal issues with Blanche, his wife, who was also an alcoholic. If I recall correctly, he himself had some issues there as well, albeit not on the scale of what Blanche was dealing with. I don't remember ever seeing Blanche, despite spending the summer program plus three additional years there. I have to wonder just how much she was Persona Non Grata at the mansion, and if so, it'd be my guess that that would have been enforced/conveyed by Joe. Perhaps there were even some passive/aggressive issues between the two of them regarding the wielding of said catechism.
Joe is innately a most persuasive snake-oil saleman cum evangelist, and his audience in the education field and in the arena of desperate parents is just not used to being confronted with such a character, let alone equipped for recognizing him for what he is. I personally don't believe it was as calculated a development as "I'm going to turn my school into a cult," but rather an outgrowth of Joe's personality ("character", if you will) coupled with some training from AA mixed in with other miscellaneous... I think he was definitely attracted to the concept of behavior modification and has less scruples about the degree to which one might employ "tricks of the trade" as, it's all for the cause and, the ends justify the means.
I could be wrong, of course. We could find out some sneaky details of an intimate friendship of his with Art Farmer from way back, for instance, and that would not surprise me one bit and would just totally blow the question of "cult", if there is still one, into the realm of assured, de post facto, bonafide, certified, and santified cultorama... amen!