Hi there.
Please refer to this topic by Rand: https://moz.com/community/q/da-pa-fluctuations-how-to-interpret-apply-understand-these-ml-based-scores
Read #2:
You've earned more links, but the highest authority sites have grown their link profile even more
Since Domain and Page Authority are on a 100-page scale, the very top of that represents the most link-rich sites and pages, and nearly every index, it's harder and harder to get these high scores and sites, on average, that aren't growing their link profiles substantively will see PA/DA drops. This is because of the scaling process - if Facebook.com (currently with a DA of 100) grows its link profile massively, that becomes the new DA 100, and it will be harder for other sites that aren't growing quality links as fast to get from 99 to 100 or even from 89 to 90. This is true across the scale of DA/PA, and makes it critical to measure a site's DA and a page's PA against the competition, not just trended against itself. You could earn loads of great links, and still see a DA drop due to these scaling types of features. Always compare against similar sites and pages to get the best sense of relative performance, since DA/PA are relative, not absolute scores.
Basically, DA/PA is relative metric, it's relative to 100 DA websites, so, if let's say facebook got +million links, and you got only +10, then relatively to FB your website DA will drop. That's why so many websites' DAs have dropped after last crawl.
To see how good/bad your DA changed, compare to your competitors.
Hope this helps.