Your domain authority ranking can drop because of several factors, like:
Getting involved in a link scheme: Your domain authority can decrease dramatically if you buy or sell links, use automated link-creating tools, or exchange links excessively.
Having lots of low-quality links: If spammy sites are linking to your domain, it could harm your score.
Having domain technical issues: Slow site speed, broken links, or other issues can also lower domain authority.
Your competitors improving their performance: Finally, your domain authority could also decrease if your competitors start performing better in the SERPs.