imo they are one of the two, or both, of the following:
1. market makers hunting for an inventory of shares.
2. traders who are tapping the bid and hoping to spook sellers who don't watch L2 and don't know how wide the spread is. if a buyer on the bid already has shares, they can sell a few into the bid and hope that someone will freak out and hit the bid for more shares, thereby filling their order. this happens constantly in pennyland.