The actual event area could only hold up to 500000 people at best but there were 1,4 million who all wanted to get there. And to get there, you had to go through a narrow tunnel. And those people were pushing to get in there while others also wanted out from the event area.
You don't need a lot of imagination to figure out that this is a recipe for disaster, especially with probably 3/4 of the people drunk or drugged up, IMO.