Domain authority isn't a Google metric—it's a third-party score from Moz that predicts ranking potential on a 0–100 scale. It drops for three main reasons: you lost valuable backlinks, Google devalued links you have, or competitors outpaced you. Backlink loss is the most common trigger. If a linking domain goes offline, removes your link, or adds nofollow tags, your DA can dip within the next Moz index update (roughly monthly). Losing even 5–10 links from DR70+ domains can knock 2–5 points off a DA score in the 40–60 range. We've seen Ottawa clients drop 8 points after a directory network shutdown wiped 30+ contextual links overnight. Algorithm updates also matter. When Google's core updates devalue certain link types—expired domains, PBNs, spammy guest posts—Moz's model adjusts to reflect that those links carry less weight. Your link count stays the same, but the quality score tanks. If you built heavily on tactics that fell out of favour (2019 PBN crackdown, 2022 helpful content pruning), expect DA erosion even without losing actual links. Relative competition is the silent killer. DA is calculated on a logarithmic curve against all domains in Moz's index. If competitors in your niche gain 50 referring domains from high-authority sources while you gain 10, your relative position slides and your DA can drop 3–5 points despite net growth. This hits local SEO hard—Toronto real estate or Ottawa legal firms often see DA swings as rivals run aggressive link campaigns. At Ottawa SEO, we track DA as a portfolio health indicator, not a KPI. A 5-point drop isn't a crisis if organic traffic holds steady. We audit the referring domain list in Ahrefs or Majestic to spot lost links, check for manual actions in Search Console, and compare link velocity against top 3 competitors. If DA dropped but you're still ranking, the issue is often just index churn or competitor noise, not a fundamental problem with your link profile.