You know, it never really says. In "The Cask of Amontillado" all we know is that Montresor has some sort of gripe with Fortunato and wants to see the man dead. Not only that, he wants to see him dead bad enough that he invents an elaborate scenario in which to entomb him behind a wall forever.
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
We don't know what the insult was that made Montresor so angry, but oddly enough, it doesn't really matter. The heart of the story is that this nobleman was so insulted by it that he wants to kill someone he considers a friend. And to entomb him behind a wall, where he will slowly starve to death and degenerate into madness? What could a friend (or an enemy, for that matter) do to you that would make you act in such a way?
Fortunato doesn't seem to suspect anything, either, willingly going into the catacombs with him. So whatever Fortunato does, he must not realize he has done it, as he does not seem suspicious that foul play is afoot.
Poe seems to do this a lot...think of "Tell Tale Heart." The only reason given for why the guy in that story wants to kill the old man is because he has a gross eye. Otherwise, he likes the old man. My point is that Poe seems less concerned with the specifics causing a lot of his stories and more concerned with a dramatic outcome.
Here's kind of a cool picture from the scene: HERE.