When someone subscribes to your channel, it should be because they have a real interest in your content and will be watching it and supporting you. To make it fair for everyone and keep this subscriber count meaningful, YouTube verifies the legitimacy of the accounts that are subscribed to your channel to prevent spam and abuse.
If you noticed that your subscriber count is dropping, here are some of the reasons why this may be happening?
- Users are unsubscribing - There are many reasons for this to happen, but some of the reasons may be that they don't feel as connected to you as they did before, because you changed your type of content or it's quality, because you don't engage with your audience, or because you don't create enough videos for your community to be active.
- Inactive subscribers - If you have subscribers that don't really watch your content, is likely that they will be removed from your subscriber count at some point.
- Suspended accounts - The subscribers were from suspended accounts.
- Spam accounts - Accounts identified as spam will be removed from your subscribers list.
- Use of services to boost subscribers - Use of these types of services, either through automated means (i.e. bots) or via a marketplace (group of users that subscribe to each other for the benefit of the group) violate YouTube's Terms of Service.
Note: For the users that are removed by YouTube from your subscriber count, they will still see you as a subscribed channel and they will continue to receive your videos in their Subscriptions feed. It's also important to remember that if your subscribers are not watching your videos, they also don't contribute to your channel's watch time, which is an important metric to join the YouTube Partner Program, and for the amount of revenue your channel generates.
To learn more about this, check what YouTube has to say in the following links: