why was the ships crew nervous at the beginning of the story?

Text evidence; exactly what the crew was nervous about isn't specified. The text simply notes their nervousness, the name they've given the island (Ship-Trap), and the fact the sailors have a second sense that the island is evil in some way.

"The old charts call it `Ship-Trap Island,"' Whitney replied." A suggestive name, isn't it? Sailors have a curious dread of the place. I don't know why. Some superstition--"

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"The place has a reputation--a bad one."

"Cannibals?" suggested Rainsford.

"Hardly. Even cannibals wouldn't live in such a God-forsaken place. But it's gotten into sailor lore, somehow. Didn't you notice that the crew's nerves seemed a bit jumpy today?"

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"Yes, even that tough-minded old Swede, who'd go up to the devil himself and ask him for a light. Those fishy blue eyes held a look I never saw there before. All I could get out of him was `This place has an evil name among seafaring men, sir.' Then he said to me, very gravely, `Don't you feel anything?'--as if the air about us was actually poisonous. Now, you mustn't laugh when I tell you this--I did feel something like a sudden chill."

Because the island has "abad reputation" , according to the sailor's and captain's previous knowledge .