Since there is no pic. and I don't know what you cooking on..... I would have to say that it could be 1 or a combination of factors, charred fat or dried out surface, did the rub have sugar in it it could be burnt sugar( does it have a bitter taste). You had the point to close to the firebox side of if it was a RF pit to close to the nose end.
I never met a Cow that I didn't like with a little Salt & Pepper! Certified PORK-A-HOLIC