@bjgrace Hi There, I was wondering if anyone would ask about that.
I do not know a simple answer. My osteo doctor did not find a cause. Nor did doing additional testing at Lani Simpson's direction provide one. Obviously there are reasons why this happens to us but "medical science" just isn't there yet, at least for some of us.

Here's my educated guesses: I had an extremely weird diet for the past 45 years due to extreme food sensitivities. One aspect of this was very low calcium intake and low intake of a number of nutrients.
I did exercise starting in 2012 or so but before being diagnosed with osteoporosis I was not using weights.
After being diagnosed I still didn't get going on exercising with weights consistently. I would do it for several months and then stop for a few months and so on.
During that time I was losing at a high rate which really got my attention so I finally started lifting weights consistently. Following a modified Belinda Beck LIFTMOR/Onero protocol but not quite as heavy a resistance. 85% of 1 rep max for 5 sets of 5 reps is what got the results in their studies. My efforts did not stop me from losing though not quite as rapidly. And it was only during the past 2 years that I did find out I could actually boost my calcium and other mineral intake without reactions.
So no simple answer to why I got to -4.1 but lots of bits and pieces. I like to think that the lifting the weights in a manner proven to work for others will work for me (and was helping me) but that other factors (some unknown) were just too strong in a negative direction. Certainly I feel so much better exercising and I haven't fractured despite some fairly interesting falls.

I'm not a clear example. The only period I was doing close to the LIFTMOR protocol with weights, consistently, for a good time period was the year where I took Evenity and made terrific gains. That was probably mostly the Evenity but I like to think that the weights and better nutrient intake, etc helped also.