Don't panic! The second follower culling
Today Instagram has given warning to it's users to not be alarmed if they see a drop in their followers and following. They announced that they are starting a second wave of deleting followers who are considered as spam acccounts, fake accounts and other people and posts who do not follow their Community Guidelines.
Personally I think this is a good thing, as having fake / disingenuous followers does not help anyone and does not add to a positive Instagram experience. Previously, Instagram would temporarily disable users who were liking, following or unfollowing too much, stating that they had reached the "limit". Your account would be locked out from such actions for 12-24 hours before it was enabled again.
Instagram has, and continues to emphasize their "community first" stance and with their service reaching an average of 300 million active users per month, they're sure to have recently implemented new automated spam security measures.
I think that this is a stern warning of Instagrams position on spammy behaviour such as follow & like spamming. If you're not careful you'll run the risk of getting your account deactivated, or even worse, banned!
You can find out more about their Accurate Follower/Following Counts here.