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 your referring domains dropped in DR
If your dofollow referring domains have lower DR, the amount of link equity that they can pass to you also decreases, resulting in your own DR dropping.
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Note: Any changes to the way DR is calculated in Ahrefs will be publically announced.
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.