Remember that this is the first time that his family have actually seen Gregor in his new, transformed state, so when he rushes out of the room to try and speak to his office manager and somehow manage to keep his job, they are obviously incredibly shocked and surprised at his appearance. His mother in particular is incredibly shocked, then terrified as she knocks over a pot of coffee and Gregor snaps his jaws at it. The office manager runs away down the stairs, but Gregor is able to distinctly hear him say "Ugh!" It is his father, however, who responds in the most violent way:
...the father, with his right hand, grabbed the cane that the office manager, together with a hat and overcoat, had forgotten on a chair and, with his left hand, took a large newspaper from the table. Stamping his feet, he brandished the cane and the newspaper at Gregor in order to drive him back into his room.
Notice how this shows a certain presence of mind. Gregor's father is the one character who is able to cope with his shock and channel it to do something that helps the situation, although this of course shows significant cruelty to Gregor.