Ann: Quarterly Activities Report, page-482

Usually those instant orders that queue after orders have been made are bots. All instant, faster than what you can place your own buy or sell.

I had a couple of different buy orders for another stock, I was first in line at the price point. As soon as I placed my order, two orders immediately went behind mine to stack the buy side. Did the same later in the day, first in line at the price point and another two orders immediately (for the same amount as last time) appeared behind mine. Bots doing bot things - programmed for different outcomes.