I did the good morning stretch today, and I was able to get lower than I've ever gotten before. I'm thinking that it may have something to do with having done RDL's yesterday and having done 1-Leg RDL's today with a weight for the first time.
I tried air squatting right after the stretch, and I didn't really have rounding of the lower back, but I still had the butt wink.
I stretch my calves and ankles, so I know they're not the problem. I don't see how tight hip flexors or adductors could cause a rounding of the back...maybe weak hip flexors. I do have a tight right hip flexor though, which makes my right glute pretty hard to activate. I don't even know how you'd stretch your adductors. Would hugging your leg stretch your adductors?
I've noticed that the tight muscles on my body are generally weaker. Maybe, I need to stretch and strengthen my hip flexors? L-sit works the hip flexors, right? I never thought of myself having weak hip flexors, but I guess that could be the case.
Also, earlier in my rehab, the RDL really did the trick in loosening up my hamstrings as well. I think it has something to do with the fact that your CNS won't let a muscle stretch to a length in which it doesn't have strength in; hence, why people who don't perform full ROM exercises tend to feel that working out reduced their flexibility.