“Might have been expected to rage” is used to mean that, in the situation, rage was a likely response. “Might have been expected to X” is commonly used in sentences where what actually happens is less expected than X. You could also write: “one WOULD HAVE expected him to rage” — I suppose this would convey a bit more confidence/probability about the expectation of rage. But really, the typical use of “might have been expected to X” is not to make fine distinctions about the degree of confidence, but rather just to set up a contrast between what was natural to expect and what actually happened.