
UPDATE #2: FOP has reopened to guests following what appears to be a false fire alarm and subsequent evacuation. So thankfully those reports of fire were untrue.
Cast Members stated simply that there was unexpected downtime. FOP has reopened to a 200+ minute wait after about 45 minutes of downtime.
UPDATE: Christy Carter Schnable posted the following comment on Blog Mickey’s Facebook thread for the story – read in full below:

We’ll be reaching out to Christy in due course to see what we can find out more.
Original Post: According to numerous reports on social media, it would appear that Flight of Passage (FOP) at Pandora – The World of Avatar in Disney’s Animal Kingdom has been evacuated fully and is experiencing downtime “indefinitely”.
Whilst guests have reported that the attraction caught on fire, we have to stress this has not been confirmed. Issues with FOP’s building fire alarm system have continued since it opened, leading to long closures in the past.
At this time, the queues have been emptied and the attraction has been evacuated, with guests being led backstage. Stay tuned to Orlando Theme Park Zone as we’ll continue to update this article in due course.
Source: Blog Mickey, WDWNT