I had indeed been using only one product to co-wash the whole time. It could very well be a sensitivity to one ingredient. The thing is my scalp was never sensitive, sore, red, itchy or anything unpleasant the whole time I was cowashing. My scalp was quite lovely in fact! This leads me to believe that it may not have been an allergy or sensitivity... as it probably would have lead to other symptoms as well. That's why I still think the over-conditioned follicle hypothesis is the most plausible explanation.
And big hugs and thanks to above-posters for your kind words! It has been pretty disheartening. I was so excited to start being "intentionally CG" and take better care of my hair that I felt understandably "betrayed" when it started falling out. At the end of the day, what works for some doesn't work for all.
Also, I remember dating some guys some years ago that started losing their hair. Back then they were all really upset but I never thought it was a big deal; it was just hair. Now that I started losing mine, I have much more compassion. It's pretty devastating. I have a lot of <3 for people who go through this.