I made some research and founds some factors why. Onions can cause swelling or irritation in your gums and teeth for several reasons:

1. Strong Compounds: Onions contain sulfur compounds that can irritate sensitive gum tissue. When you chew them, these compounds can trigger a reaction that leads to inflammation.

2. Acidity: Onions are mildly acidic, which can contribute to gum irritation, especially if your gums are already sensitive or inflamed.

3. Allergic Reaction: Some individuals may have an allergy or sensitivity to onions, leading to localized swelling or discomfort in the mouth.

4. Bacterial Growth: Onions can leave residues in your mouth, which may promote bacterial growth if not cleaned properly, potentially leading to gum issues.

If you notice that onions consistently cause swelling in your gums, it might be best to limit their consumption and consult a dentist for further advice.