My theory or should I say opinion is that it is a way to make a quick few ticks by someone who is trading very large size. Sweep it 5 ticks, get all the scalpers/spreaders stuck and then buy or sell it back from them when they get out. IT never really seems to have any follow through after the sweep like you think it would. IF it only happened once I would say it was just some random order, however it seems to happen everyday or every other day at this point. To me it seems to be a new trading scenario for someone in the market.