I was actually on this exact flight in the BusinessFirst cabin of UA904 (sitting in one of the first few rows right by the galley) and witnessed most of the incident. Just to confirm, the passenger never seemed to act unruly and actually remained very level headed throughout the entire ordeal. The flight attendant darted behind my field of vision into another part of the BF cabin, came right back up and went to the other flight attendants. She said something to the effect of "There is a customer in the back of the cabin taking pictures of us while we work." She then disappeared for a while (presumably to go talk to the captain?) and I actually didn't think much of it. There were several FlyerTalkers on the flight (I saw by their luggage tags) and I just assumed it was a member of that crew documenting for a trip report (amusingly, all of us ended up at the Grand Hyatt Istanbul). I'm actually shocked to read what ultimately transpired and see the actual photo. In fact, I'm happy to post MY photo of the flight map showing the flight path from EWR-IST which I took from my seat, uploaded to social media and was not thrown off the plane for taking. The gentleman sitting next to me in BF actually did the exact same thing, in fact.

The event seemed so innocuous that I really just moved on without thinking twice. This is really just bizarre to me and while I don't know what happened in many private conversations, I can assure you there really was no ruckus or disturbance to the BF cabin beyond the initial outburst by the FA.