Why are my lower back problems gone?

In the FS video Coach says "... monstrous lower back problems ...". Why?

I am asking because my lower back problems appear to have gone since I started to do the Stretch series on a consistent basis about six months ago (all three of them, not just FS). I am not complaining, just curious.

The problem I used to have was that if I spend a lot of time walking/standing, after I get home and sit, I sometimes would get an intense sharp pain in my lower spine which would last for days and in the worst case much longer. When calling medical advice, they just told me to exercise more. But I was already exercising doing Foundation. I have been doing Foundation since it came out, so it's not the strength training which made the difference. In fact, some of the mobility exercises in F1 would sometimes trigger the issue until recently.

Just curious. I don't like doing the Stretch program. It's boring, a bit uncomfortable at some points and takes a lot of time, but it looks like it improves my quality of life, so I'll keep doing it.

I think some lower back problems are actually the result of tight flexors and some others are a result of tight hamstrings. FS helps with both. I'm sure that Thoracic Bridge also helps.

It is more than likely the combination of all three programs rather than one. You have discovered something that works so even though you may find it boring at times you have to keep doing it for ever! I usually listen to podcasts when I stretch so the time passes.

I get a lot of what I have heard non-medically referred to as spinal decompression in the front split series, particularly in the standing pike variants. Anecdotally and without anatomical basis (yet consistently) I have noticed this improve my general ease in my own skin, including my mood/patience. Perhaps that's what they meant by 'exercise more'

Yes, anyway, I am happy that whatever it is has helped, and I will for sure be doing this forever.

I am far from an expert, but from my own experience, everything in the posterior chain is/can be the culprit for lower back pain. Guys above mentioned tight hips (notably, massaging the outside of the glutes [medius?] works wonders, as well as lunges that work the hip flexors) and tight hamstrings. In addition, I was amazed one day when I rolled out my calves with a softball, and it really loosened up my back.

Point being, everything is connected. That's good or bad news depending on your situation.

The calf muscles are innervated by nerves coming of the low back, as to our all the leg muscles for that matter. When you massage muscles you stimulate a lot of sensory receptors in the local tissues, which feed back to areas of the spinal cord that are also related to spinal structures and muscles. 

The effects of massage and manual therapies have many neural effects, some reflexive, some involved in the perception of pain and muscle tone. This is what I do daily with my patients to help alleviate all sorts of painful conditions