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
someone on r/cth really pushed me into a veil-piercing moment when they pushed back on my probably NYT-derived perspective on Maduro in venezuela. I did just the tiniest bit of fact checking and realized I'd been straight up lied to. from then on I searched for counter narratives regularly and often found them similarly or better supported by the evidence
Plus sexuality/gender stuff has made me a lot more commie tbh. Trans people largely being so desperately poor and liberals basically just sneering at them or at best pitying them, and the rise of assimilationist trans media figures, and then learning a better history of the lbgtq rights movement(s) made me realize none of this was new and that assimilationist gays did the same shit 20+ years ago. It really reinforces the fact that it's one struggle and our solidarity has to be absolute.
Bernie getting ratfucked helped too I guess but honestly I wasn't quite as invested in that campaign as some. It was just like "yeah duh, this is the bare minimum and it would still improve countless lives"
Plus a long time interest in open source/free software helped a bit