The cause of the explosion was not immediately clear. Officials linked the explosion to some 2,700 tonnes of confiscated ammonium nitrate that were being stored in a warehouse at the port for six years.
6 years of natural decomposition of the A N will have caused the material to heat up enough for it to combust.

It is classif​ied as an "energetic material", one that produces heat as it decomposes, similar to the way heat is generated by rotting material in a compost heap.