The actual reasons behind it are surely as varied as the users themselves... ;)
Might produce an interesting little dataset for the "data science folks" out there to add a "State your reasoning for this Flag:" requester to the flagging process.
It's nice to think about how this kind of data could be used, but with anything like this we have to consider the UX impact of making it a higher-effort interaction, as well as the observer effect: https://en.wikipedia.org/wiki/Observer_effect.