Hi there!
Jo from the Moz help team here.
While it can't be hard to say for certain why your PA decreased I can offer some guidance for how you can research what has happened here.
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 we have combined into a single, predictive score.
Your score can decrease if there was a reduction on the number of equity passing back links to those pages, have you lost any links or disavowed any links recently?
It's important to note that both Domain Authority and Page Authority are on a 100-point, logarithmic scale. So you authority could decrease as our index grows and we locate more equity passing links to other sites - even if you gain more backlinks.
We recommend keeping track of how many linking domains you have in the discovered and lost section of Link Explorer or the Links section of your campaign, as this will be a quick way to confirm possible reasons for an increase or decrease in the score.
To get the most out of the authority scores we recommend comparing with competitors’. By measuring the distance between these scores, and tracking the change over time, you'll be able to identify if you're improving relative to the sites that really matter — your direct SEO competitors. We also recommend keeping track of how many linking root domains you have from index update to index update, as this will be a quick way to confirm possible reasons for an increase or decrease in the score.
I hope this helps, please do let me know if you would like more clarification or have any other queries.
Jo