Someone (or a few) probably just flagged your ad. It doesn't take too many flags before it's pulled. Could have even been done by one person using an auto-flagging program. Autoflagging programs are available for purchase, and they make it really easy for one person to get a posting removed from Craigslist. Autoflagging software connects to a proxy server, flags the posting in question, switches proxies, flags the listing again, and repeats that process until the listing goes down. Anyone looking to bump competitors off of Craigslist can do so with minimal effort. Youtube has plenty of videos showing how.