prepositional phrase as a modifier

[Newton]

1) "As it happened" reports an event whose occurrence was previously in doubt or not expected, often in contrast to what was expected.

Q) What does the underlined part describe/modify in the previous sentence? Does it modify the verb "report"? Or does it modify the noun "an event"? Or the clause "whose occurrence was previously in doubt or not expected"?

[Alan]

Although somewhat redundant, it appears to be intended to modify 'occurrence' (= 'AND IS often in contrast...').