Doctor Who: 12 Cool Details Revealed In Steven Moffat's Blink Commentary
11. This Is Why The Weeping Angel Threw A Rock At Sally
After exploring the house for a bit, Sally pulls back some wallpaper to reveal a message left by the Doctor, specifically for her: duck!
Doing as the message says, Sally bobs to the ground as a large rock whizzes by above her head. Clearly, the rock was thrown by the Weeping Angel stood in the garden, but during the commentary, Moffat said that he constantly gets a lot of questions about this moment, and so, he addressed it in a bit more detail:
"A lot of people have asked me ‘who threw the rock?’ The Weeping Angel threw the rock. Now, because of the particular M.O. of the Weeping Angel - and its inability to move while you see it - it is of course to their advantage to render their victims insensible."
Admittedly, the episode doesn't make it fully clear why the Angel wants to knock Sally out before sending her back in time - after all, aren't they meant to be lightning-quick?
But then again, considering that the Angels in this story aren't exactly at the height of their powers, they probably just wanted to make her an easier, safer target.