People are unfollowing you
People unfollow others because of multiple reasons. One of the most significant reasons for the decrease in followers is people unfollowing you. (See Why do people unfollow me on social networks?)
Instagram is deleting spam accounts
Instagram has an excellent feature that keeps the service clean by removing spam accounts every few days. If there were an array of spam accounts that were following you, you would see a sudden fall-off in followers.
Deletion of Inactive accounts
Various accounts are inactive, and Instagram might consider them as spam too and delete them; hence if such accounts were following you then again, that would make your followers less.
Your purchased followers
Various services sell followers; for doing this, they provide people with a plethora of fake accounts that Instagram deletes when they begin with a cleanup.
You didn’t follow people back
If you do not follow people back, they might unfollow you. Thus, if you do not follow people back, then there are chances that you might see a curb in your followers too. (See 13 Reasons Instagram is bad for your Mental Health)
Always make sure that you do not post anything offensive or against anyone’s culture, religion, or tradition as people might feel insulting, and they stop following you.
A bug increased your followers
Mistakenly once it happened that a bug increased the number of followers, then Instagram announced that the follower count would get back to normal once the bug is fixed.
Instagram targetting purchased followers
Instagram is very particular about real and fake accounts; hence, if they think that purchased followers are decreasing the social network’s quality, they might delete them even if they look like real accounts.
Why your profile picture doesn't get a lot of likes on Facebook | Why some people don't like my profile picture on Facebook?