Herbicide exposure is my thought. Where did the mulch come from? It may have herbicide residue in it. Sometimes rather potent, persistent herbicides are used to kill weedy trees and brush, and then the dead plant material is cut down and chipped up and may end up in a mulch pile. Grass clippings used as mulch can also have herbicide residues. I know that one chemical used in lawns is Tenacity (active ingredient is mesotrione), which causes weeds to turn pinkish or white in color.
