Well, I had two thoughts on why someone was there to overhear the prophecy...
1) the most obvious, I think anyways! It was jsut coincidence. He was a guy (I'm assuming) who just happened to be int here at the time, who worked for Voldie, and overheard something and went running to Voldie withthe info (or 1/2 of it!!!)
2) Someone was trailing DD, and since he wasnt' doing anything really important, he didnt' bother to shake them off, and they overheard and went running to Voldie.
Personally, I like 2 better!!!