Why? Because it's a supercut of airplane crashes and other inflight disasters (zombies, snakes and so on) taken from Hollywood movies.
As creators Screen Junkies point out: "Ever notice how they cut out airplane crashes during inflight movies? Well this supercut is the complete opposite."
And it probably goes without saying, but: you might not want to watch it if you have a fear of flying.
(And for film fans trying to spot all the movies featured - they are: World War Z, Fight Club, Hook, Final Destination, LOST, Planes, Trains, & Automobiles, Flight, The Grey, Almost Famous, Final Destination 5, Mega Shark vs. Giant Octopus, Donnie Darko, Snakes on a Plane, Air Force One, Turbulence, Iron Man 3, Passenger 57, Home Alone, Twilight Zone: The Movie, Breaking Bad, Superman Returns, Executive Decision, Cast Away, Wayne's World, Airplane!, Die Hard 2, Con Air, Knowing, Alive.)
(Via Laughing Squid)