Welcome to the Q&A Forum

There will be 2 reasons: 1st one is you lose a backlink or backlinks from that certain website which causes to increase in your website's DA. and 2nd is there is some kind of bug all you need to do is wait for few days but if it still remains the same it means 1st is your main problem of decreasing the DA.

I am also facing same issue on my Website, If you found any solution Please let me know.