There's never one single cause to something like this. The other poster mentioned shipping, and German attacks on American shipping was a big part of it. The Zimmerman telegram was the final straw. But there were concerns about loans to England and France and there was natural affinity to for England that Americans tend towards in foreign policy. There were other ideas that France and England were more similar to democracies that Germany and Austro-Hungaria.

So, it was a confluence of all those factors, some being more important like shipping. Some being harder to quantify, like natural sympathy for the English.