The novel several times makes the point that Vito never uttered a threat, not even to enemies, and he instructed Tom to do likewise. A wise policy. But, Michael, in Sicily, gratuitously threatens Sr. Vitelli in his cafe when Vitelli wants to know who he is and what he wants with his daughter :

"I am an American -- hiding in Sicily..My name is Michael Corleone...There are people who'd pay a lot of money for that information...But then your daughter would lose a father......instead of gaining a husband."

That's a hell of a way to introduce himself to the father of the girl he's just been smitten with, and who he's depending on to get an invitation to visit his daughter. More to the point: it's a self-fulfilling prophecy. How could he keep that info fro spreading all over a small Sicilian town where everyone knows everyone else, ]and they're probably related, too? The threat made him sound (perhaps intentionally) like a big-time American gangster.

He also agreed to a very public wedding (while "hiding in Sicily"), with the whole town invited. Naturally, people were going to ask Vitelli about his daughter's bridegroom. What was he going to say, "He's some cretino from somewhere who fell for Apollonia while he was taking a nature walk with these two lupara-toting bird-watchers"? Small wonder that word got back to Barzini, who got to Fabrizio.