Elon Musk just clarified what happened: "Saw pressure fluctuations on Falcon boost stage liquid oxygen tank. Want to be super careful, so pushing launch to Thurs."
I was watching the stream. Immediately prior to the final hold (more accurately "Hold! Hold, Hold, Hold!") there was a clearly abnormal burst of fuel vented from the midpoint of the vehicle, in two or three bursts. Happened several seconds after they pulled back the support structure. My eyes widened at that point. "Is this normal?!" I thought, and was glad to hear the hold command!
https://twitter.com/elonmusk/status/405134552458620928