Here is a rough and highly incomplete description of the the development of instruction. The purpose is to highlight the step-by-step evolution, showing how one step was indebted to what went before. (I am writing this from memory--I could make minor errors.)
1. In 1951, the Scripps Institute of Oceanography in California created scuba classes in response to the death of a diver in Berkeley. They made it all up out of nothing, learning how to dive themselves first, brainstorming what could possibly go wrong, figuring out what to do if it did, and designing a way to teach it all.
2. In the later 1950s, Los Angeles County sent Al Tillman to Scripps to take the class and learn how to teach it. He brought it back to Los Angeles, and that became the Los Angeles program.
3. Scuba was also being taught by a wide variety of people across the country, including in YMCA classes. There was no established way to do it. Al Tillman (and others) decided to take the Los Angeles program nationwide, although he could not, of course, use the local tax money that had been funding it. He hosted a gathering of various instructors in Houston in 1960, and that led to the creation of NAUI. Tillman became NAUI Instructor #1. The core of the program was the original Scripps program, but they openly allowed the instructors from other backgrounds to add their own stuff. Many of those instructors were originally trained in the military. One of those additions that was not part of the Scripps program, for example, was harassment--removing masks, shutting off air, etc.
4. Tillman tried to maintain the Los Angeles structure, but without a tax base, he was dependent upon donations to replace it, and much of that came from Skin Diver magazine. When that relationship ended, they struggled to survive. They focused on university students for their customers, because their already-paid tuition made the courses essentially free. In 1965, they decided they were overextended nationally, and they retreated to California. In doing so, they canceled a major instructor training session scheduled for Chicago. The Chicago branch of NAUI was furious, and they responded by forming PADI.
5. [This part may possibly come just before the end of the last part--not sure.] Los Angeles County was frustrated that so many of their students quit diving shortly after being certified. In an attempt to get them to continue diving, they added a course--Advanced Open Water. They hoped that by introducing people to a variety of kinds of diving, their interest might be piqued. NAUI adopted the program right after that. (I am not sure when PADI picked it up.)
SUMMARY: The original Scripps program in 1951 was the core of instruction throughout the formative years of scuba instruction, and most of what happened during those years was a matter of tweaking. Instructors coming from outside of that tradition, especially instructors coming with military background, brought additional stuff that many older divers may have experienced and assumed were mainstream activities.