Question

What made the boys follow the footprints

Answer 1

The boys followed the footprints because they were amazed at the footprints appearing from nowhere. The footprints were of a man who was bare feet; they wondered who could be bare feet in such a cold weather in London.  

The footprints were appearing from nowhere, following one after another; the boys kept following them; however, soon they became fainter and fainter and disappeared completely.