The reason being is that the German "U-boats" were attacking British trading ships in order to try to blockade and starve Britain into surrendering. But when Hitler saw that they weren't starving because American ships were allowed to pass, he started ordering them to attack U.S ships. When there was an outcry by the American people and threatening to start a war, she temporary stopped but then decided to start the attacks again in hopes of defeating Britain before America declared war. But it may have been a mistake because the Germans ended up sinking a ship that was carrying hundreds of people to Liverpool, including well over a hundred Americans that was sunk by a German U-boat. The ship carrying them was called the Lithuania and by sinking American ships like that, it caused the American people favor against Germany which help prompt them to go to war/support Britain even more. So in other words, he ordered the German U-boats/subs to sink U.S ships because they were supplying Britain with food, ammo, and ordinary supplies.
