Can a mild allergy to pineapple lead to the same reactions/allergy to fresh grapefruit? why would it occur?

Cross reactivity: Those fruits share protein properties in their fibers, so it's very common if someone is allergic to a certain fruit can well be allergic to other fruits

OralAllergy Syndrome: Oral allergy syndrome may occur in up to one-third of individuals with seasonal allergies and results from a cross-reactivity between seasonal airborne pollen proteins (i.e. tree, grass, weed) with similar proteins that are found in various fresh fruits and vegetables like pineapple. Symptoms include itchiness, tingling and/or swelling of the mouth, tongue and throat, immediately after eating

Not usually: Although some fruits share common allergens (melon/banana, avocado/banana), pineapple and grapefruits are not known to cross react. However you may be at a higher risk of developing allergy to other citrus fruits from grapefruit allergy.