Why does Eurylochus distrust Odysseus when he wants to return for the rest of the men, what is his specific reasoning?
I assume that you are talking about what happens in Book 10. In that book, Eurylochus leads the men to Circe's house. All of them except for him are turned into pigs. He then goes back to find Odysseus. Odysseus says he is going to go to Circe's house and free them men from the magic spells.
Eurylochus is afraid of Circe and does not want to go back. Instead, he wants the two of them (him and Odysseus) to just escape on their own. What he tells Odysseus is that he will never save any of the men -- he will probably get himself killed or turned into a pig himself. Here's the quote:
I know you will not bring one of them back with you, nor even return alive yourself