Poor weather (it was a wintry day in February) and pilot error. (The pilot was not experienced at instrument-only navigation.)
Source(s):
http://en.wikipedia.org/wiki/The_Day_the_Music_Died

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The Big Bopper was piloting the plane when he noticed that Richie Valens was wearing Chantilly Lace. This caused him to,"feel real loose, like a long neck goose." When the Big Bopper tried to make his move on Valens, Holly stepped in and said,"That'll be the day when I die!" A long physical confrontation ensued, similar to the final scene in Bloodsport. It is a well known fact that Buddy Holly was an accomplished street fighter in the bare-knuckle boxing circuit. I believe it was this experience that gave him an advantage over the Big Bopper. After wearing the Bopper down to the point of exhaustion, Holly straddled his broken body and looked him straight in his bruised and bloodied face. As he opened the bay doors and prepared to throw the Bopper out he uttered,"Get the hell out of my plane!" It was about this time that the three of them realized no one was flying the plane, but it was too late...And that's the day the music died.
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