My guess is that either:
1) the info was leaked by an insurance company
or, more likely
2) a good algorithm pieced together all sorts of details about your life and made the deduction and sold that deduction to the group.
You need to realize that every little purchase you’ve made for decades had been recorded via your credit cards and thrown into a software blender, analyzed and out come all sorts of fairly accurate guesses about you, your life and your health.
Your prescription purchases are theoretically confidential (ha ha) but if you use a credit card, that info is spread all over the place.
not to mention that anybody over the age of 60 probably has some kind of back pain....easy thing to do is cast a wide net....