Did it just happen naturally?
Yes it did. I made up my mind not to obsess about sleep like I did with DD (nearly drove myself round the bend with her) and just tried to wait things out. It was tough and some nights when he was in the middle of a long NW, I was sure I had done something wrong but within a few days things would be better again. I can't remember if I said this already, but I just fed DS for pretty much all NWs because it was easier. I was just starting to think about sleep training (at 16 months) and he started STTN on his own.
That said, feel free to post in NWs if you want to some other ideas.