Had that happen to me a couple times, though it was a smaller sub, and the mods knew I was a regular. I posted, forgetting I was in one of those subs, to tell a right-wing troll what I really thought of him, and the autoban went down.

Though in my case, it wasn’t a big deal - I appealed, worded my appeal in a friendly way, and I was reinstated in 5 min.

I think in this case, the autoban was put in place simply because of the nature of the sub - it was a leftish political sub that got brigaded and trolled regularly by Reddit’s resident chuds.