Like a moment that seemed insignificant and unimportant at the time, but looking back was in fact a pivotal moment in pushing you towards radicalization.
For me probably being introduced to Guitar Hero at my friend's house in like summer 2007. At the time it was just another day, but looking back I ended up falling in love with those games, I ended up being introduced to The Ramones and even more importantly the Dead Kennedys through them, which caused me to get into punk music and resulted in me adopting very critical attitudes towards larger American society, attitudes which later grew into anarchism and then Marxism
Two things actually. My first grade teacher explained (in gentlest terms possible) that some people opposed Columbus Day because he was an enslaver and initiated the genocide of the native americans. She also talked about why people took issue with thanksgiving and why doing that thing where you ululate while putting your hand over your mouth was deeply racist. "Huh, it seems like something fucky is going on" is the impression she left on me.
The other thing was that at some point in time when I was a kid I started to question the purpose of money and why there were rich and poor people, my mother cautioned me to not think or talk like that because "that's called communism, it's bad, and you'll get beat up at bars if you say that to people." This struck me as rather odd at the time but I did ultimately drop that vision of a better, more just world for a good long while.
The path to leftist thought is similar to my gender journey, where after many misdirections and much confusion I have come back to the place I started from. That initial "this seems strange, but this feels right" was the truth all along.