"The Cask of Amontillado" by Edgar Allen Poe is a great story, but it's full of ambiguity. Poe does not come right out and say what Fortunato has done to Montressor. He does give us some clues, and we can make a pretty good inference. Whether we're right or whether we're wrong, there is no doubt that Montressor feels the wrong was egregious. Let's consider the following:
"Come," I said, with decision, we will go back; your health is precious. You are rich, respected, admired, beloved; you are happy as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill and I cannot be responsible. Besides, there is Luchesi"
This allusion to Fortunato's wealth and standing in the community could very well be the cause of the rage. Even the name "Fortunato" implies that wealth might be the issue. Perhaps, at one point, Montressor's family was wealthy and respected, but now it no longer is. This seems to me the most logical conclusion.
Enotes has some good information.