To get more information about AQS read our blog post: "Why You Should Look Into Audience Quality Score."
Unfortunately, agencies are used to making wrong assumptions regarding the real value of each influencer based on an ER level only. It is quite a major pitfall for brands, since Engagement Rate is not an indicator of its comments, likes and followers quality.
The easiest way is to check who tends to like and comments on your posts on a regular basis. Are those people real or perhaps your ER can simply come from a bunch of suspicious accounts and mass followers who boost an ER in an artificial way.
If you've recently spotted a gradual decline in an AQS level, it's in your interests to tackle this issue in the most effective way by simply checking your Followers Quality. The usage of so-called follow-unfollow methods (Mass Followers) to artificially boost your followers count. At the same time the implementation of such inauthentic methods in practice will bring the Followers Quality level down quite drastically.
If some anomalies, such as abnormal growth, are identified in the Growth metric, then there were most likely used some methods used to inflate the followers numbers artificially.