It's looking for movement in the frame. I was playing around with it and I noticed and once I settled into a picture, it would jump into HDR+ no matter if it was bright, dark, or mixed. I did notice that it would drop out when focusing or tracking a moving object, so I held it up and waved my hand in the side of the viewfinder and, yep.. no HDR+
Now, this is pretty shrewd to be honest. I've played with HDR+ quite a bit over the years, and it ALWAYS takes the better shot. And as they've sped it up a considerable amount, the number of situations in which it works well has increased quite a bit. The only situation where it doesn't work as well is if there is movement in the frame... say a kid running across a soccer field. It looks like Google has pretty much embraced that and changed it so that Auto HDR+ triggers HDR+ mode in every situation that it doesn't register much movement. It's extremely clever in my opinion.