Comedy comes from surprise, either from the breaking of a (moral) rule (e.g. a taboo) or from something unexpected (“boo!”) (or both – man-falling-on-banana-skin is unexpected and makes us laugh, naughtily, from the suffering of another).

And in Tragedy the hero’s fall must also be a surprise, at least to him, and must come from a combination of two reasons: if it’s caused by fate or ‘the gods’ it’s in effect a random event, unexpected; and if it’s their own fault it’s from their breaking a moral rule.

So both arise either from the unexpected or from moral transgression.