Many many things. Very loud music, cold/infections/viruses etc. Personally mine burst due to a ear infection/virus, because of fluid behind my ear building up. Its completely painless and nothing to worry about but there is a small amount of discharge fluid.
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Though it is very difficult to cause rupturing of eardrum, physical impact, High frequency sounds, health issues, genetic defects are most probable causes.
Because ear drums take in vibrations and if it's busted there's nothing to take them in and turn them to sound.