Bruce, I wonder if your water temps got too hot for the pond weed or maybe a good rain muddying up the water a little bit with cloud cover for several days was enough to start the pond weed die off. One thing I have noticed is that when weeds start to die off there is self feeding loop that propagates and feeds the die off sometimes. The chlorophyll leaches out of the dead vegetation turning the water a brown tea color very quickly, the reduced water clarity then feeds more die off with both the remaining weeds and any good algea bloom that is going on.