Can someone please help me understand what's going on in this order book? This is a screenshot of the XRP/USDT order book from Poloniex today. I have often noticed that there is a sell order and a buy order of almost the same size (usually quite large) and I think it is the same person doing it (either manually or using a bot)

https://images2.imgbox.com/43/6b/AB0xvRnA_o.png