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
