A logical explanation sans real ghosts is probably that Ichabod spent the better part of an entire evening gathered with men who told fantastic ghost stories and legends about the area of Sleepy Hollow. His imagination just gets the better of him as it has so many times before when he's reading until late in the afternoon and walks home alone. When the party finally breaks up and Ichabod leaves the estate for home it is the "very witching time of night". As Ichabod is known to do, he begins to allow his imagination to run wild on his ride home as he reflects on the tales that were told that night. As he approached, "the very place where many of the scenes of the ghost stories had been laid. . ." he sees a gnarled scary looking tulip tree, the tree actually played character to a local legend and many people has spoken of seeing odd things take place near this very tree. He thought is he whistled at approaching the tree that it might calm his nerves, but as he approached he saw a white apparition and heard a terrible moan. It could have been real, but if you don't believe in ghosts, then logically his mind just got away from him.