1 People are unfollowing you
People could unfollow others for various reasons. One of the most popular reasons for the drop of followers is people unfollowing you.
2 Instagram is deleting spam accounts
Instagram tries to keep the service clean by deleting spam accounts. If many spam accounts were following you then your account might experience a sudden drop in followers.
3 Deletion of Inactive accounts
Some inactive accounts might be considered Spam accounts by Instagram and so they might get deleted as well.
4 Your purchased followers
Most services that sell followers provide people with fake accounts that are very likely to be deleted when Instagram initiates a clean up. See why Instagram is popular.
5 You didn't follow people back
Many accounts unfollow people if they didn't follow them back. If you didn't follow back a large number of people then your follower count might drop. See why Instagram is bad for your mental health.
6 Offensive post
An offensive post , such as a racist post, could motivate many people to unfollow you at once.
7 A bug increased your followers
Instagram once announced that a bug resulted in increasing follower count and that the follower count will be reduced once again after the bug is fixed.
8 Instagram targetting purchased followers
Purchased followers might affect the quality of the social network and as a result the Instagram team might try to delete them even if they seemed like real accounts.