It's possible that the additional therapy has made a difference. Seeking various treatments to keep good function is often advised. Studies haven't been done to show what each kind of therapy does to maintain our health.
It isn't possible to find out what worked for you without a control group (patients with the same injury who weren't treated). Was it the combined therapy at the time of your injury? The chiropractic and massage therapy later? Or just good luck?
Future studies are needed to find out what works best and why. Maybe patients who aren't treated at all have the same long-term results as those who are in treatment. This kindof information is hard to come by. Researchers aren't usually allowed to put patients in a "no treatment" group.