For those of you comparing your DA over time, it's worth reading about what DA actually is:
https://moz.com/learn/seo/domain-authority
It's effectively Moz's informed guess at how well your site will rank. It takes into account three broad things: the signals that your site give out, what others in your niche are doing and what Google is looking for.
As you can only ever control 1 of the 3 sets of things, your DA will fluctuate over time and you'll be very hard pressed to figure out why.
DA can still be useful, as the page I linked to suggests, as a way of comparing your site to competing sites, but it's not really worth looking to 'improve' it, as such, as so little of it is under your control.
Rather than focus on these rather abstract metrics I'd suggest writing more content that's relevant to what your intended audience, optimise your site's performance (speed), optimising on-page content for key words (make good use of headings, titles, alt attributes and so on) and promoting your site.
Do this and the DA should look after itself