This article takes a pretty good dive into the science of seasoning, but the long story short is, polymerization is basically always (slowly) happening in oils. "Drying" oils polymerize "better" than others, and polymerization is accelerated by things like heat, light, and most significantly the release of free radicles (aka smoking).
When you cook oils with your cast iron, some of those oils will polymerize and bind to your existing seasoning.