There are a whole host of reasons as to why there may be an emergency landing of a flight. However, not all of them are as serious as you may have initially feared because there are moments where it is human error that is to blame rather than any technical issues.
Take a flight back in 2016 as a prime example. There was an attempt by an individual on the flight to make a complaint regarding the conduct of another individual. This flight in the US had the potential to take a bad turn if, in their opinion, something was not done.
But what could have incensed the individual so much that they had to go ahead and pass a handwritten note to a member of the staff? Of course, you may have expected it to involve some kind of terrorist threat and the silent note was so as not to draw attention, but that is not the case.
Instead, the note mentioned how they wanted the staff to deal with an individual sitting some 10 to 12 rows in front of them. Their crime? Having the most horrific smelling farts that they had ever experienced. Yep, they wanted the plane to pretty much turn around so that this individual could be removed from the plane thanks to their flatulence. Nothing was done about it, so you kind of hope that they filled the plane with their stench for its entire duration.