May be too much sugar. It also may be due to a smothered fire. Do you keep the exhaust vent fully open? Do you get bluish smoke?
To answer the sugar question, cook some ribs with no sugar beside some with the rub you are questioning.
Jim - Another transplanted Texan
Former KCBS CBJ
Large and Medium Big Green Eggs , Black 18.5" WSM, Blue Weber Performer - Stainless, Green Weber OTG Kettle , Brinkmann SnP Pro, and a Stainless UDS. One retired Portable Kitchen grill.
Red Thermapen, Maverick ET-732, EdgePro Apex Sharpener.
Avatar is the original 1951 Weber Kettle