My boyfriend's followers count in instagram increased from 300 to 1300 overnight. How's it possible?

When I asked he told me he don't have any idea, how's it possible? I'm pretty sure overnight noone can gain so many followers (1000). Is he lying to me that he doens't have any idea.
My boyfriend's followers count in instagram increased from 300 to 1300 overnight. How's it possible?
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