"Lo" is a direct-object pronoun, i.e. the receiver of the action. Because the thing is question is doing the occurring, you would need a subject pronoun; however, the subject pronoun for inanimate objects is often omitted in Spanish, leading to the phrase as it is written.
I believe it should be "cuándo" as well. It serves as a pronoun in this sentence. Otherwise, it would mean something like "we do not exactly know after it happened."