Hi DundeeLaw...could it be a water situation? I do believe that like all willows, this one would want a consistent water supply and may react to the widely varying rainfall and temperatures by dropping leaves. Have you checked for insects or damage to the bark/trunk? I don't know if yours is a grafted tree or not, but you may want to check for damage to the rootstock if it is a graft.
Take a trowel and dig out soil at the root area to see how dry the soil is. you will be surprised how much water we can throw on our plants / soil only to find it has either ran off the top of the soil no where near the plants / tree roots or it has evaporated faster than light in the heat.
if you find no evidence of insect damage or disease at the graft, then I would suggest the problem is lack of water, Themoonhowl has mentioned the amount of water these plants do like to keep healthy and my favoriteeremedyy for the lack of water is to dig out a few holes around the root area and insert up-side-down clear plastic juice bottles that you have cut the bottom from, once the bottles are in place and you have backfilled the holes, you then get the hose and fill the bottles up full a few times, this way all the water gets to the roots where it is needed, also good way to add feed if need be, I leave the bottles in place for a couple of years just to make sure the tree / shrub is growing OK, after it has settled and put on new growth, if you wish, you can remove the bottles.
Hope this helps with the watering IF that's the problem. Good Luck. Weenel.