Who is buying followers?
If you haven't heard of the popular web crawling site SocialBlade.com you might want to check it out. SocialBlade.com is a great tool many marketing specialists use to track YouTube and Instagram user's statistics. You can track a users performance which includes growth, posts, followers and estimated future projections.
There are usually trends. They will come in large amounts like 40k in 1 day. More than likely they will lose most of those followers gradually throughout a few days, weeks or months depending on who they bought from. Take a look at the example below.