Answer:
Because he knows that he is being sent to the planet of Kanamit to be devoured.
Step-by-step explanation:
In "To serve man" Kanamit and the other countrymen arrive on planet Earth with the promise of helping the planet and solving all the problems that humans cannot solve alone, such as hunger, violence, poverty, corruption, among others. They really solve these problems and say that they want nothing in return other than the gratification of helping other peoples and the friendship of humans.
Grigori is a human and does not believe that, he thinks that the aliens have a plan behind all this charity that they are doing with the earth. Then he begins to study the language of the aliens and discovers that they want to devour humans. Knowing this, he is terrified when he finds out that he is on the next transport to Kanamit's home planet.