Without seeing the horn I can't say for sure, but it is possible that part of the horn in the affected area has delaminated. Horns grow in layers and sometimes when excess or prolonged heat is applied to them a layer will kind of "bubble up" from the other layers. You see this sometimes on horns that have been polished before you get them. They are polished on a big buffing wheel which produces heat which in turn can affect the horn layers by causing delamination of the layers of horn.
I know of no cure for delamination after it has occured except to try and rescrape the affected area back down to size and hope that you don't break through the outer layer of horn.
BTW, a delamination might be almost impossible to see if the throat is stained very dark.
Randy Hedden