Who originated the dangerous avocado-pitting technique?
I sincerely doubt any particular chef or "popular show business personality" is responsible for this, though perhaps one or more such people have done this at one time or another.

Many people are used to taking other fruits in one hand and a knife in the other and preparing the fruit for eating.  I distinctly remember my father showing me how to core an apple with a pocket knife when I was about five years old.  Yes, I believe there was a warning about not stabbing yourself with the point, but people commonly use paring knives all the time with fruit held in your hand to peel things, take unedible bits off (as in hulling strawberries), and sometimes to remove pits and seeds.

I think many people who are unfamiliar with avocados attempt similar maneuvers. However, the avocado pit is much larger and more difficult to dislodge with a knife than with many small fruits.  There is also a smaller proportional distance between the pit and the surface of the fruit than many other fruits (again due to its size).  Simultaneously, the avocado flesh is much softer than many fruits people are used to dealing with, thereby making it much easier for a knife to accidentally slip completely through it than with, say, an apple when coring.  Finally, the skin of an avocado is what I'd call "deceptively hard": when you try to slice into an avocado, it may seem quite firm compared to the interior, but it's quite easy for a knife point to slip through the skin from the interior to the outside (particularly when one is applying leverage to try to dislodge a large pit).

So, you have people doing similar things with a knife to what they might do with an apple or a peach, except they end up exerting more force for the large pit while the knife slips through the flesh like butter, and the knife goes awry.

I don't think it's a great mystery why this happens.  One might equivalently ask why bagel slicing sends people to emergency rooms.  (It's one of the most common pathways from the kitchen to the ER.)



EDIT: Well, I've been downvoted -- not sure why, but I'm going to assume because I didn't cite sources.  So, I'll try to bolster my point.  (Pun intended.)  Here you go: 

A recent study estimated approximately 8900 avocado-related hospital visits in 2018.  The study notes: "The oldest person to visit the ER for an avocado-related laceration was 75, and the youngest was 8. Most of those hurt by avocados were, in fact, millennials."  So, there's a wide range of ages reported, making it seem less likely that all such injuries occurred because of bad instruction from a single source.  I've skimmed at least a dozen articles on avocado-related injuries and have not seen a single mention of any celebrity chef or other "popular show business personality" as the source of the bad knife technique.  (To the contrary, many articles cite authorities who give better techniques.)  And then you have an explanation offered by a doctor in this article regarding bagel injuries:


  Maybe it's just their shape. "Any spherical food, like tomatoes and onions, and particularly bagels, people tend to hold and cut in their hand," Dr. Smith-Coggins says.


Indeed, a decade ago, in another study the top five foods related to knife/finger injuries were (in order): chicken, potato, apple, onion, and bagel, each of which also results in thousands of serious injuries.  As avocado consumption has increased about five-fold in the U.S. in the past couple decades, it seems they're the new spherical food causing injuries.

As a final note, it is commonly recommended by many chefs to use the blade of a knife to remove an avocado pit (as here, for example, which also shows the somewhat dangerous practice of scoring the avocado in your hand after depitting).  This pit technique apparently does cause some injuries, particularly as people struggle to get the pit off of the blade of their knife.  (The more I read about the stuff, the more weird ways it seems people could get injured dealing with avocados, so I'm not sure "avocado hand" is caused by only one type of mishandling.)

In any case, I have yet to find a similar example of someone famous telling people to stab the pit with the point of a knife while holding it in one's hand.