What happened to Sirius's body when he died?

It said in the book that his body went behind the veil in the Department of Mysteries in Harry Potter and the Order of the Phoenix and in the movie, he just kinda floated away. Why didn't he just fall to the ground? That's what Cedric Diggory and Dumbledore did when they died. I am just confused about that. Is it just because they were near the veil?