Germany and the US had been in a condition of extreme tension not just because of the war in Europe but it's spill over into the Atlantic.

U-boats had sunk several US ships and killed citizens. The same complaint of unrestricted submarine warfare had drawn the US into WW1 was back.

By 1941 US Navy ships were escorting transports on the Western side of the Atlantic, then handing the convoy over to the Royal Navy. Germany saw this as undeclared aid on sorely stressed Britain while the US said it was enforcing the rights if neutrals. By the end an undeclared shooting war was on in the Atlantic, the Rueben James was a destroyer that was torpedoed and sunk shortly before Pearl Harbor.

Then Germany did the US a favor and officially declaring war first on the US, instead of the other way around after Pearl Harbor.

EDIT: As for the reason the US and Allies focused on Europe first it was actually decided on before the war. US internal War Games and the Rainbow Plans affirmed the thought that Germany was the more dangerous foe. While the ABC Conferences in Spring 1941 began to line up US and Allied thinking about the topic. Then just after Pearl, Churchill and the top brass rushed over to Washington for the Acadia Conference which confirmed Europe First as the guiding strategic principle of the war.