What's sappenin' with this tree?

We spotted this funny stuff coming out of this willow tree in our grandparent's backyard. At first Sister Agent #1 thought it was honey. Sister Agent #2 thought it was sap. Looks like Sister Agent #2 was correct. However - we still don't know why its coming out of the tree.

The sap is gooey and sticky but looks cool! The only research we came up with is something might be wrong with the tree - some sort of pest infestation. Uh oh Poppy you better do something about the willow tree!!!!