smb, it would move the ask up not down. Sellers seeing an unusual order suddenly appear think that there is a big buyer coming in and will try to force them to change their bid to something higher than the original and then will hit that number instead. It gets them a few more bucks. Quite often it is all bogus, maybe an MM trying to make a market, but I have done the same by putting a large AON order in and have seen the ask back off. If the order gets removed, the ask immediately goes back to its original position.
Its all part of the market games. No one knows if it is a real buyer or a MM so the speculation causes the B&A to jockey around, everyone suddenly takes notice, and sometimes it triggers off some trades.