According to Eric Hunsader of Nanex, a Chicago area firm that tracks market data, the strange activity is coming from a an automated trading program managed by a "dark pool" (a group of unidentified large investors), though they could also be "fat finger" errors entered by a humans.
In Pandora, for example, one order was entered at $10.4340 and it should probably been entered at $10.0434. The first strange moves took place at 10:02:20 AM.
This activity is impacting dozens of stocks (here's a full list, called MiniKnight) and the orders entered are all off by subpennies to 3 decimal places.
Also, Hunsader added, all this activity is coming from the same place.
Now why did this happen? "There's no way of knowing wy unless you ask the person," he said.