Why does Scrooge yell at Bob at the beginning of A Christmas Carol in the offices?

Because Fred visits Scrooge to wish him a "Merry Christmas." Scooge true to form rejects pleasantry and tells Fred to keep Christmas in his own way. Fred tries to give Scrooge positive points of Christmas which Bob Cratchit claps his hands in agreement with. Scrooge turns on Bob saying if he hears anymore from Bob on the matter of Christmas he would be not be in his position after it. Meaning he would dismiss Bob.