Here are a few reasons why your DR may have dropped.
1: Other sites did a better job on linkbuilding
If you didn't lose any backlinks, and yet your DR still dropped, this could be a reason.
Think of it this way: when a DR‐100 website gets more backlinks, we can’t make it DR‐101. So instead we push all the other websites down by 1.
That’s a very basic explanation of why you might see a drop in your DR while no backlinks were lost.
2: You are losing high-quality dofollow links
Domain Rating shows the strength of a given website’s overall backlink profile. It's measured on a logarithmic scale from 1 to 100, with the latter being the strongest.
That means that your DR will drop if you're beginning to lose high-quality links.
Do check your Referring Domains report to investigate what domains with dofollow links you lost at the time of decline and see if you can fix it.
3: Some dofollow link from any of referring domains turned to nofollow and that domain stopped passing rating
4: Some of referring domains lost rating itself
Note: If there is a change in our crawl algorithm, please be assured that we will definitely inform you.
Note: Please, mind that Ahrefs Rank (AR) is based on Domain Rating (DR), so reasons for decline of DR explained above apply to those of AR decline respectively.