This is just speculation but i found out that megalodon was in direct competition with a monster whale, megalodon was not the baddest boy on the block, a whales intelligence and extra senses could have made the difference . Modern Orcas kill and eat great whites with relative ease could competion with dwindling resources with Livyatan melvillei have led to the extinction of megalodon?

 

http://en.wikipedia.org/wiki/Livyatan_melvillei

If I recall correctly, there was also a change in the habits of other whales at about the same time as the extinction of the megalodon. I think they changed migration routes, as well as some species becoming extinct themselves, and megalodon could not cope with this as it was a very specialised hunter. Starvation may have contributed largely to there demise.

 

With your hypothesis, when did Livyatan melvillei become extinct? Megladon is thought to have died out something like 2 million years ago. Based on what I have read on wikipeadia, the two species for sure coexisted, but the Livyatan melvillei became extinct a little before that. (I am not expert and I am sure there are plenty of 'error bars here')