I just saw this as well for the first time today. Before today I hadn't seen this, and I installed both 85 and 91 right when the manual installs first came out. It seems to be pretty random. It's not often, and I can't find a rhyme or reason for it. Maybe it's just that I'm opening an app with my phone lying down or slightly tilted, but I'm not sure.
I also noted it takes a few seconds to recognize it and flip back to portrait even though the phone is completely upright. Mine usually fixes it automatically though. I don't have to back out. Try tilting your phone sideways and then upright again. It should correct itself automatically after a few seconds. Also, is your phone sitting on the table or something when you're opening stuff?