Domain Rating is decreasing of my website

Domain authority is just Moz scoring system of the quality of the backlinks, I guess you have been building poor quality links. Remove them is the answer and stop doing it in the future. 

However, Google doesn't use Moz's scoring system so providing your rankings are good then it doesn't really matter does it.

Few reasons why your domain rank dropping may have dropped.

• Getting more backlinks for Other sites even if we do better job on linkbuilding, can be one reason for domain rank drop.
• If you are losing high-quality dofollow links
• Some dofollow link which we link to may turned to nofollow and that domain stopped passing rating
• Some of referring domains lost rating itself
• Changes in the crawl algorithm