Hi Dan,
DA is a fantastic metric to compare domain strength and judge their overall backlink profile and ability to rank. However, it is not a good metric to Trend to show domain improvement. Simply put, the metric data set is different with every Linkscape update.
If you go to the Open Site Explorer homepage, you will see that the recent crawl contains 104 billion URLs. That's an amazing number. Their next crawl could theoretically cover 107 billion URLs. You may have hundreds of additional backlinks found in those extra 3 billion pages crawled. Your DA goes up, despite you doing nothing to the website. Most likely, your competitors DA goes up as well.
The next update may only crawl 103 billion URLs. It doesn't crawl 100s of pages with your links. Your DA goes down. So do your competitors.
Simply put, it is a sample size of backlinks. It is great for comparing one domain to another, due to the data being based off of the same sample size. It's great for competitor insights and analysis.
When your DA goes down, that doesn't necessarily mean your site is less authoritative. If you still have links pointing to your website, you don't lose that equity because Moz had a smaller crawl. Remember, DA is a marketing metric, but not a Google ranking factor.