Why is the island called Ship Trap Island?already answered in group
The island has been given this name because of how many ships get shipwrecked there. It appears that the seas around the island are somewhat treacherous to being with. However, General Zaroff also helps cause some of the shipwrecks because his lighthouse shows a channel where there are actually rocks. Here is a good quote on this topic ("them" refers to the sailors that Zaroff hunts):
"Sometimes an angry god of the high seas sends them to me. Sometimes, when Providence is not so kind, I help Providence a bit.
Zaroff then shows Rainsford the light that is supposedly indicating a channel. So this line implies that the area is dangerous on its own. So maybe the name of the island had already been given to it before he came. But he certainly creates even more of a trap for ships.