For me, I'm pretty sure it came from childhood trauma. Perhaps there's a biological component, as well -- my mother has extreme memory traits/talents too, but hers are not applicable to personal autobiographical memories. She ended up with more of a photographic memory, but only for books, reading, and school. She also suffered a lot of childhood trauma, which confounds my data points here, I know.

It certainly could be an evolutionary trait, that gets activated in the modern era because of stress. Pure speculation on my part here, though. I would love to read an actual study with real data about these phenomena.