The person or detective who described Wilson as a madman when looking at Gatsby's body was in the right place at the right time, and people therefore took what he said as authority.
Someone with a positive manner, perhaps a detective, used the expression "madman" as he bent over Wilson's body that afternoon, and the adventitious authority of his voice set the key for the newspaper reports next morning. (ch 9)
The word “adventitious” basically means to take advantage of something, or for something to happen more by chance than by design. When the detective describes Wilson as a madman, the papers basically take the idea and run with it.
Naturally with a mysterious public figure like Gatsby, people will be curious and the newspapers will be all over it. They were basically listening until someone said something worthy of print, and as soon as they had somethin good they used it.