Some of them might be bots indeed, every platform has issues with bots.
But DEV has an onboarding feature that suggests authors that might be interesting to follow based on the tags you selected on sign up. This is maybe why you have a lot of followers that are new users.
Here is a post from @ben
about this feature:
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.