I wonder if this is an attempt at manipulation. The robot creates a sense of great activity by placing many limit sell orders above the market, then lowers the bid. Others might think something is happening and sell. A short term downward price change might follow.
The price change might be small in most cases but sometimes a larger price break may follow. Perhaps it's that bigger price break that the robot is intended to trade.
I am reminded of the Jesse Livermore story about skinning skinners, the bucket shop operators, from Reminiscences Of A Stock Operator, page 54 in my copy of the book. In this story, Mr. Livermore places limit buy orders under the market price, then manipulates the price lower. When the price values rally Mr. Livermore sells at a profit. Maybe the picture below records a millisecond version of the same idea.
<img src=http://www.elitetrader.com/vb/attachment.php?s=&postid=2916005 \img>