Mainly for the same reason the heel wears the fastest at the back of the heel. And maybe some of what you describe plus where the heel strikes the leather is going to get compressed (and then open up a little) with each footfall. And if the leather is flanky...as the excessively loose and coarse fibers indicate...that compression and decompression is going to be accentuated. That will, in turn, tend to "shed" the wax that protects the heel leather from rain water, etc.. And so it's a cumulative effect--a vicious circle.
And even though it is pronounced in this case...probably because of the poor quality of the last layer in the stack...it's a common occurrence to one degree or another. Even on the best made shoes, leather heels and outsole edges can benefit from a little attention now and again.
PS...and on edit...the method I describe above is fundamentally how I do it when I make a shoe or boot. Some aspects are changed to accommodate the consumer who doesn't have access to a shop and professional dyes, tools, etc., but the basic concepts are "in there."
Makes sense. Thanks for the explanation!
When you burnish, do you do it dry then add wax or add wax then burnish?