I lean strongly toward the third of Lyonel Perabo's explanations. Burying money was the easiest way to keep it from being stolen. read more
First off generally pirates did not bury treasure. Most of the loot was perishable goods so they had to be sold quick anyway. The top pirates which count at most 20 that could afford robbing more would store it for at most a couple of months and no boby traps just guards sometime. read more
This is very uncommon - in fact, it was more common for a Chinese pirate to do this than a European or Carribean one. Furthermore, legend has it that each time Captain William Kidd buried treasure, he killed one of his crew members and threw them into the pit, because he believed that the dead pirate's spirit would protect the riches from treasure hunters. read more