allergy next to bactrim but not beside sulfonylurea, what is the component within sulfa drug grounds allergy?

sulphur! (also spelled sulfur)
It is an allergen in susceptible individuals. Those patients should avoid drugs which contain sulphur.

Hope it answered your Q
Bactrim is sulfamethoxazole and trimethoprim two different salt that work as bacteriostatic drugs, avoiding bacterial reproduction.
Bactrim causes different side effects:
Gallbladder irritation
headache
severe throbbing on legs
loss of appetite
rash
an allergic antipathy (difficulty breathing; closing of your throat; swelling of your lips, tongue, or facade; or hives);
unusual bleeding or bruising
yellow skin or eyes.
fatigue
dizziness
nausea
vomiting
decrease appetite
diarrhea
weakness
increased sensitivity to the sun.