Domain Authority can fluctuate over time simply because of the dynamic nature of the web. Links are constantly being added, removed, and changed, so as the backlink profile of a domain changes, so does the domain authority. Sometimes, backlinks are removed, which could result in a drop in domain authority. Other times, new backlinks are added, and this could increase the DA of the site. Every backlink influences the DA score, however small, so the cumulative impact of all the backlinks combined can affect & change the score over time.