I have a theory - that thrush makes all infected tissue very fragile, so
vasospasm and whatever causes the shooting pains along the duct pathways
happens, and cracks can appear at the base of the nipple. Healthy skin has
enough elasticity to accomodate the skin stretching blood flow, milk flow,
etc.  I'm not a doc - is this a reasonable explanation?

Linda Smith, also a believer in ductal thrush after seeing David Mathis'
slides of yeast penetrating intact epithelial and endothelial tissue in 1991