Why was Equiano’s “astonishment” converted into “terror”?

This is Equino's first introduction to the slave ship. He was astonished by it and then terrified by it when forced to go on board.

These filled me with astonishment, which was soon converted into terror when I was carried on board. I was immediately handled, and tossed up to see if I were sound, by some of the crew; and I was now persuaded that I had gotten into a world of bad spirits, and that they were going to kill me.