Ever since I upgraded to the latest version of Android 7.x on my nexus 6p I started experiencing this issue.
At first I thought it was related to meeting notifications but that was not it. Then I thought maybe when I make a call or receive a call. That is not it. I have seen it do into dnd when all I am doing is browsing an app.
This is on :
Android running 7.1.1
Let me know of you need more info.