A decrease in your Domain Authority (DA) despite your efforts to improve it can be frustrating. Several factors could contribute to this situation:

Algorithm updates: Search engines like Google regularly update their algorithms, which can affect how they evaluate websites and determine DA.

Competitor activity: If your competitors are actively improving their own websites, it could impact your relative position and DA.

Technical issues: Issues with your website's technical performance, such as slow loading times or broken links, can negatively impact your DA.

Content quality: Despite your efforts, if your content quality doesn't meet the standards expected by search engines or your audience, it could affect your DA.

Backlink quality: Low-quality or spammy backlinks pointing to your website can harm your DA. It's essential to focus on acquiring high-quality backlinks from reputable sources.

Changes in search trends: Shifts in user behavior or search trends can impact your website's visibility and, consequently, your DA.

To address this issue, continue focusing on best practices for SEO, regularly monitor your website's performance, and adapt your strategies as needed. Conduct a thorough analysis of your website and its performance metrics to identify any specific areas that may need improvement. Additionally, consider seeking guidance from SEO professionals who can provide personalized advice based on your website's unique situation.

This happen when the backlinks of your website dropped. You have created new backlinks to boost up your Domain Authority. Here's the complete guide for you.

First mistake: to thing you're doing all right.

If DA is going down is because something it's not right. To think that Google made the mistake rating your page is not the right thought. The other answer saying: "SEO is a black box and sometimes weird things without explanation happen". That's bullsh***. Maybe you cannot see it, but the explanation is there.

Once said that, what I can tell you is that maybe you have been using some practices considered black had without your knowledge. Pay special attention with the external links. Well used they can bring you authority but wrong used it can hurts. Learn when to use dofollow or nofollow and how to use the anchor text.

Check the user experience: loading speed of your site, average bounce rate, average engagement time. All those parameters tell Google if users are enjoying your stuff. So check if they are in good rates.

Sometimes there's some things out of your control that can affect your DA. For example if you're sharing the host, maybe one of your "host neighbours" is doing black had, or is a porn page or violence content site. In that case you all have the same IP so you all got penalized. You can check who are you sharing host with. There online free tools that tell you that in seconds. Another option could be someone from the competition trying to hurts you in purpose. They can link to you from bad reputation sites using dofollow links. This could hurts. Check your income links in your search console maybe you can find out something suspicious.

Hope it helped

Any black hat techniques have been carried out on your domain? Have you checked on spammy links and used disavow tool?

Can you share the domain?

DA is a long term game, I've seen websites go down and then spike up. SEO is a black box and sometimes weird things without explanation happen.