'Update: Instagram has confirmed that this is a bug, the cause of which it is actively investigating. In a statement the company told me "We're aware of an issue causing a change in account follower numbers for some people right now. We're working quickly to fix this".
I'll update this story when the issue is resolved along with any additional information Instagram provides. The original story is below for reference.
Instagram users have noticed that they have fewer followers today than they did yesterday. It seems that Instagram has conducted a cull of some accounts leaving some users annoyed at their new lower follower counts. Some big names and large companies are affected.
Social media consultant Matt Navarra has compiled some numbers on Instagram's biggest accounts which have, by virtue of their follower count, lost the largest numbers. Katy Perry, for example, lost 900,000. Professional influencer Kylie Jenner has been hit hard too, dumping 2.5 million followers in total.
Sports fashion brand Nike lost a large number of followers too, dropping from 84,600,00 to 83,500,000 - that's 1,100,000 accounts in total.' - via FORBES