The Ammonium Nitrate at Beirut Port was there due to a complex legal dispute. A story of the legal wranglings is on http://shiparrested.com 

On 23/9/2013 a ship called the Rhosus, flying the Moldovian flag, sailed from Batumi Port, Georgia heading to Biera in Mozambique...

The Rhosus was carrying 2,750 tons of Ammonium Nitrate.

En route to Mozambique, the Rhosus got into technical difficulties, forcing it to enter Beirut Port. Upon inspection of the vessel by Port State Control at Beirut, the vessel was forbidden from sailing further.

Most of the crew were repatriated shortly afterwards. The Rhosus was abandoned by her owners after charterers and those shipping the cargo lost interest in the Ammonium Nitrate.

The remaining crew on the Rhosus and Master quickly ran out of stores and provisions.

Various creditors came forward with claims and three arrest orders against the vessel were obtained.

Efforts to get in touch with the owners, charterers and cargo owners to obtain payment all failed.

In the meantime, the Master and crew remaining on board were in jeopardy.

Diplomatic efforts were attempted to have
the remaining crew repatriated without success.

On application to a court, a judge eventually allowed the crew to return home, leaving the Ammonium Nitrate on the Rhosus in Beirut port.

Owing to the risks associated with retaining the Ammonium Nitrate on board the Rhosus, port authorities decided to move the cargo into a port warehouse.

It's not clear what's happened to the Rhosus, but the Ammonium Nitrate should have been disposed of years ago at auction.

This is a photo of the Rhosus taken on 19 April 2014, just a few months before it was impounded at Beruit.

It gives you an idea of the size/bulk of the Ammonium Nitrate that exploded yesterday.