The answer is Preformed antibodies in the recipient's blood will bind and clump the donated cells.
Every person’s blood type is determined by the antigen on their blood cells. If a person is of blood group A, then they have antigen A in their blood cells. Additionally, the same person will have antibodies to the alternative antigen, i.e in our case the person with blood group A will have antibodies for antigen B. Therefore if a patient with blood group A is transfused with blood group B blood, then the antibodies in the patient will bind to the transfused blood cells and cause agglutination.