Why is a single footprint so terrifying to Crusoe?

It happened one day, about noon, going towards my boat, I was exceedingly surprised with the print of a man’s naked foot on the shore, which was very plain to be seen on the sand. I stood like one thunderstruck, or as if I had seen an apparition. I listened, I looked round me, but I could hear nothing, nor see anything; I went up to a rising ground to look farther; I went up the shore and down the shore, but it was all one; I could see no other impression but that one. I went to it again to see if there were any more, and to observe if it might not be my fancy; but there was no room for that, for there was exactly the print of a foot—toes, heel, and every part of a foot. How it came thither I knew not, nor could I in the least imagine; but after innumerable fluttering thoughts, like a man perfectly confused and out of myself, I came home to my fortification, not feeling, as we say, the ground I went on, but terrified to the last degree, looking behind me at every two or three steps, mistaking every bush and tree, and fancying every stump at a distance to be a man. Nor is it possible to describe how many various shapes my affrighted imagination represented things to me in, how many wild ideas were found every moment in my fancy, and what strange, unaccountable whimsies came into my thoughts by the way.

When I came to my castle (for so I think I called it ever after this), I fled into it like one pursued.