What does glucosamine have to do with shellfish allergies?

Shellfish shells. Glucosamine is manufactured from the shells of shrimp, so contamination with shrimp proteins might cause a reaction. However, most people who are shellfish allergic are allergic to the muscle of the shrimp, and the amount of the muscle protein in the glucosamine product is small.
Not much, but... Many preparations of glucosamine are prepared from shellfish, so there is a risk that it may be contaminated with the allergens that cause shellfish allergies. The major allergens in shellfish are chemically different from glucosamine, so there is little risk from pure preparations. Most supplements are not pure, however, so it may be safest to avoid it in cases of severe shellfish allergy.