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If this was a used refurbished irrigation sprinkler system, such as one that was outdated for use in the US, but still salvageable for down island use, it is quite possible that the previous owner had been using fertilizer injection in the irrigation system, meaning that fertilizer residue would still be in and on the sprinkler equipment.

Also, many irrigation sprinkler distributors also sell the fertilizer for fertilizer injection irrigation in bulk quantities to become a one stop shop for their customers irrigation equipment needs. Warehouse cross contamination is another probable cause.

There has been extensive criticism of the homeland security industry related to nitrogen and urea related compound detection systems. Namely they can detect the compound class, but can't differentiate between cat piss and fertilizer from explosive material.

The lesson learned here is that the US government at Federal , State and local level, as well as private industry put under mandate to inspect cargo, has wasted billions of dollars on first generation technologies, and have wasted billions of dollars of tax dollars and increased fees to end users, due to the post 9/11 spending spree on the homeland security industrial complex.

(That was long winded.)
All we can hope for is that the initial spending spree is criticized so the manufacturers of homeland security equipment are forced to pay for R&D and made to give the public a better return on investment. False alarms cause millions of dollars in damages, this situation shows how correct critics have been on this issue.