Willows, #1, are short lived trees. They are subject to innumerable pests, including willow aphid, spider mites, carpenter worm, canker disease, wetwood rot, drought die-back, etc.
I would ask the same questions: did your trees get PLENTY of water during a drought or did they go through a period of drought stress? How old are the trees? Do you see large holes in the bark (pinkie size)? Did you have a late freeze in your area last spring? Are there any oozing areas on the trees, especially that foam and have an odd smell? All of this stuff can have an effect on an accurate diagnosis.
My answer is NOT to plant willows...