Why a guy who didn't interest in me for so long suddenly take an act?

I met him in 2017, i was into him. But he always gave me a cold response. Since i moved to other city i tried to keep in touch but he's clearly not interested. So i move on.

But suddenly this month he follow me on IG but doesn't ask anything, just there .. i haven't say anything too.

Do you know why someone act like that?

Sorry bad English. I hope you're doing fine, stay healthy!