Well, in the initial draft (actually second draft, I think?) of Godfather II, there's a scene that wasn't filmed where Pentangeli is talking to one of the Rosatto brothers and says pretty much that they caused him the heart attack because of how they ran their territory ("Everything with them is whores, and.. eh.. (something in Italian?).. dope!") I assume that when Clemenza was given the Corleone's old territory the Rosatto Bros. were under him, and he wanted them to run things a certain way. When they didn't, it caused him a ton of stress and, ultimately, a heart attack.

That's just my interpretation.