2. Spam accounts are disappearing
Instagram makes an effort to delete spam accounts, many of which might be following you (and may have been influential in growing your audience in your early days of using the social network).
As these types of fake and spammy accounts get reported by Instagram users they get closed down – which can mean your follower count does a nosedive too.
3. Your content has changed
Let's say you posted a lot of make-up-clad selfies over a consistent period of time and you grew an audience that really liked them. Then, you started posting landscapes or photos of your cat.
This is an immediate turn-off for Instagram users. They will unfollow others when something new is posted that isn't consistent with the reason they followed in the first place.
4. You post too much
The general rule of Instagram used to be one post per day. Even that now seems too frequent. When your followers are constantly seeing you in their feed, they get bored of you.
What's more, Instagram gets bigger and bigger (it reached one billion users this June) so people are always looking for new people to follow, and old people to unfollow. When you post something undesirable for a certain follower, it's a prompt to delete you from their feed.
5. You've been shadow banned
This is a controversial (and highly speculative) measure that Instagram apparently uses. Shadow banning means Instagram could limit your exposure on its platform and make it harder for people to see your posts via hashtag searches and other ways of promotion. This would inevitably suppress your ability to reach new people.
Potential reasons for being shadow banned are because you've used an automated software service or bot to grow followers, you like too many photos or you follow/unfollow too many people (there are various hourly limits depending on how old your account is), or you've been reported by other users as having an account that violates Instagram's terms and conditions.
One final way to get shadow banned is to use hashtags that are blocked or marked as offensive (you can find out what these are by searching a hashtag's most recent posts and you'll see a note saying they are hidden because they don't meet Instagram's community guidelines).
If you abuse Instagram in any of these ways, stop it. It's probably why you're losing followers. Delete any offensive photos and banned hashtags, don't overuse the platform, and remove any access to your account that third-party autobots have.
Instagram has wised up to what you're doing, and you're being punished for it.