Most likely cause is it didnt get cold quick enough or kept cold between caping and getting it to the taxidermist. I believe its a bacteria or something similar thats a result of warmth on the hide.
I think the specifics on how bad it is could help you decide. Took my first shoulder mount I've done since an adult to a reputable Houston taxidermiat and they didn't even tell me about it. There were only a few spots its noticeable and the rest seems fine, though I don't mess with it of course.
Aside from that, its figuring out logistics of a new cape if this is that important of a deer to shoulder mount.