For me, "serendipity" in writing comes from introducing plot elements without knowing why, and then they become Major Drivers all on their own.
A scene from current fic: a teenage boy is having a spiritual moment while admiring a big redwood tree on a mountain bike trail, and out of nowhere steps this grungy old man, staring at him. He never says anything, just stares for awhile. Then he nods towards the tree, gives the boy a salute, and disappears into the ferns again.
I couldn't resist putting that scene in, and I had no idea why at the time. It just seemed like something that SHOULD happen. Salute Man, as I call him, may not even ever show up again. But then I realized a connection he might have with another character - a char who was NEEDING a connection, but I hadn't decided who/how yet - and BAM: Major Plot Driver. Lots of loose ends just fell into place. The scene was perfect, including his appearance and the tree and the salute. How did that happen?