What led Adolf Hitler to order German submarines to attack American ships?

Due to the US wanting to prevent any future wars, they passed a series of laws known as the Neutrality Acts (http://www.encyclopedia.com/history/encyclopedias-almanacs-transcripts-and-maps/neutrality-acts-0), wherein it would simply make it a lot more difficult to get involved in whatever conflict (thus why the US was referred as a sort of "sleeping giant" before they joined the war).

They later (21st September, 1939) implemented the Cash-Carry policy (https://en.wikipedia.org/wiki/Cash_and_carry_(World_War_II)), which ended up replacing the Neutrality Acts. The Cash-Carry allowed for the US to sell supplies to allied nations, only if they came to pick up the supplies themselves (and of course paid in cash).

The Cash-Carry act was then superseded by the Lend-Lease act (enacted in March 11th, 1941), which allowed for the US to sell military supplies to the Allies. (https://en.wikipedia.org/wiki/Lend-Lease)

This of course angered Hitler, as Britain could hold out for an extended amount of time (and the looming possibility of the US joining against them), which led him to send the Kriegsmarine (or the German Navy, it is a branch of the Wehrmacht), under Karl Dönitz, and Chief of Navy Erich Raeder (who was replaced by Dönitz in 1943), to the Atlantic (with the famous German U-boats taking center stage, this also led to their nickname as "the wolves of the Atlantic".) In order to then of course attempt to choke out Britain. This was known as the "Battle of the Atlantic", which lasted up until 1945. (https://en.wikipedia.org/wiki/Battle_of_the_Atlantic)

If you want more info:

Footage (3:38 min)

(42 min)