Why do people follow and then suddenly unfollow on Twitter?

Bots, mostly. They follow you in hope that you'll follow them back, and if you don't follow them in 3 days (for example) they unfollow you so their followers/following ratio doesn't raise flags.
While they follow you, you posted interesting and they unfollow you when you're posted boringly, I think ;)
Follow-unfollow is very old technique for growing Twitter account and followers.....so it's nothing personal , they didn't even look at your content.
Because they have ran out of space to follow other people. That's how twitter following works.
Someone might've started to follow you expecting something entirely different to what you've been tweeting. It's nothing personal.
An old trick that does not work anymore because it has been spammed to death.

You follow someone, unfollow and hope the person follow you by himself which limits your number of followers per day and makes red flag dissipate for a potential ban.

It's a bad strategy in 2021, same as everything else with an organic strategy with social medias.
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