Lol, I read this thinking it was about flags (the ones you fly on poles). Which in itself might be the reason people associate the term "flag" with marking something for future attention as it waves hello at you!
But no, in blogs and forums, flagging has a bit more of a serious connotation. At WikiTree G2G it is in relation to inappropriate material or abusive comments.
But elsewhere (on the www) it can also be a signal for maintenance, deletion, additional research etc.