Where did all these rocks come from?

It seems like every other month I read about yet-another "city sized" rock making a flyby past the planet. What really concerns me is most of these things are announced when their pass by is immanent. This can only mean one of two things: Option 1 is that the things are really flying out of the darkness and suprising us mere months before they miss (or altnatively hit) our poor planet. Option 2 is that our astronomer types tracking these things DO know theyre heading our way years in advance, but don't know if the rock is going to hit us or not until its merely a couple of months out.

Neither option fills me with confidence.