Why it is said that cephalosporins can cause hemolytic anemia?

Autoimmune reaction: Cephalosporins interact with red blood cell membranes. The body can sometimes produce antibodies against cephalosporins that also interact with the surface of red blood cells. These antibodies thus activate the body's immune system to attack its own red blood cells, causing hemolytic anemia. This is a rare but severe side effect of cephalosporins.

They can, rarely: Like a lot of other medicines, cephalosporins do interact with the red cell membranes and occasionally produce new proteins that some folks make antibodies against. Usually this is a non-problem ("Coombs positive") but occasionally enough red cells get damaged so there's actual hemolysis, anemia, and the systemic stuff from damaged red cells.