Hi there! Tawny from Moz's Help Team here.
Aw, man! Sorry to hear about your DA dropping. You're right that it appears to have happened for everyone, across the board. I think I can help explain. The reasoning behind this can be hard to pinpoint without the help of an SEO consultant or the specific web designer for your website. Domain and Page Authority scores are both calculated using Moz's Ranking Models. In essence, we take a lot of rankings data from the search engines (by running queries) and then try to build a predictive scoring system using our own on-page analyses and Mozscape link data to construct an algorithm that will effectively reproduce the search engines' results. Our current accuracy hovers in the 70% range, but over time, we expect to improve.
Once we have a ranking model (which we internally call "uber"), we can create scores that best approximate the combinations of all our page-specific link metrics or domain-specific link metrics (removing the keyword-specific features like anchor text, on-page keyword usage, etc). These scores represent the model's query-independent or non-keyword-based ranking inputs.
In simple terms, Domain Authority is our best prediction about how content would perform in search engine rankings on one site vs. another. Page Authority answers the same question for an individual page. Both are amalgamations of all the link metrics (number of links, linking root domains, mozRank, mozTrust, etc.) we have into a single, predictive score.
It's important to note that both Domain Authority and Page Authority are on a 100-point, logarithmic scale. Thus, it's much more difficult to grow your score from 70 to 80 than it would be to grow from 20 to 30.
It is also important to note that if the higher authority sites that your site is seeded from drop in score, it will ripple down to all of the other links branched off from the very top. A good place to start is to compare the DA from the linking root domains from the previous index. Also take a look at your competitors scores to see how much they have dropped as well. As our indexes grow in size, there are more links that are included in our calculations. I think this is a big part of why we saw drops in Domain and Page Authority this time around - more links included in our calculations will skew the scores a bit lower.
We recommend keeping track of how many linking root domains you have from index to index, as this will be a quick way to confirm possible reasons for an increase/decrease in the score. For an in-depth discussion from Rand, check out this article: https://moz.com/community/q/da-pa-fluctuations-how-to-interpret-apply-understand-these-ml-based-scores I've found this article to be most helpful when I'm trying to determine why Domain or Page Authority scores may have fluctuated so much, especially when it's across the board like this.
Here's some places to really delve into what is going on:
http://moz.com/blog/whiteboard-friday-domain-trust-authority
http://moz.com/blog/googles-algorithm-pretty-charts-math-stuff
http://moz.com/blog/whiteboard-friday-domain-authority-page-authority-metrics
Here are some good resources to help you take a look at the factors.
http://moz.com/blog/whiteboard-friday-domain-authority-page-authority-metrics
http://apiwiki.seomoz.org/w/page/20902104/Domain%20Authority
http://moz.com/blog/whiteboard-friday-domain-trust-authority
I hope this helps!