Check for incoming links and see if there's any pattern. Check referral traffic and see if any bot traffic is landing on your website or not. Try the following steps:

1. Create a filter in analytics
2. Block spam IPs
3. Block bot traffic to land on your website.
4. Create quality backlinks
5. Find out pages that are performing lower in SERP i.e. 5,6,7 or 8,9th positions. Optimize them and push them for better CTR.

Once you are done, submit sitemap and wait for some time, I hope this will help you out recovering the lost DA as it works for me.

Regards,
Ravi Kumar Rana
TheSEOGuy